[llvm] [docs] Document `-mtriple` option in opt command guide (NFC) (PR #213942)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `amdgpu-offload-build-only` running on `AMD-bb-w-02` while building `llvm` at step 4 "annotate".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/226/builds/17564

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 4 (annotate) failure: '../llvm-zorg/zorg/buildbot/builders/annotated/amdgpu-offload-cmake.py --jobs=32' (failure)
...
[273/1107] Building generated/OffloadAPI.h.gen...
[274/1107] Building generated/OffloadEntryPoints.inc.gen...
[275/1107] Building generated/OffloadFuncs.inc.gen...
[276/1107] Building generated/OffloadImplFuncDecls.inc.gen...
[277/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_wait_release.cpp.o
[278/1107] Building generated/OffloadPrint.hpp.gen...
[279/1107] Building include/OffloadErrcodes.inc...
[280/1107] Building CXX object compiler-rt/lib/dfsan/CMakeFiles/clang_rt.dfsan-x86_64.dir/dfsan_chained_origin_depot.cpp.o
[281/1107] Building include/OffloadInfo.inc...
[282/1107] Generating OffloadAPI.h
FAILED: offload/liboffload/API/OffloadAPI.h /home/botworker/bbot/amdgpu-offload-build-only/build/runtimes/runtimes-bins/offload/liboffload/API/OffloadAPI.h 
cd /home/botworker/bbot/amdgpu-offload-build-only/build/runtimes/runtimes-bins/offload/liboffload/API && /home/botworker/bbot/amdgpu-offload-build-only/build/bin/clang-format -i generated/OffloadAPI.h.gen && /usr/local/therock-tools/bin/cmake -E copy_if_different generated/OffloadAPI.h.gen /home/botworker/bbot/amdgpu-offload-build-only/build/runtimes/runtimes-bins/offload/liboffload/API/OffloadAPI.h
/bin/sh: /home/botworker/bbot/amdgpu-offload-build-only/build/bin/clang-format: Permission denied
[283/1107] Generating OffloadEntryPoints.inc
FAILED: offload/liboffload/API/OffloadEntryPoints.inc /home/botworker/bbot/amdgpu-offload-build-only/build/runtimes/runtimes-bins/offload/liboffload/API/OffloadEntryPoints.inc 
cd /home/botworker/bbot/amdgpu-offload-build-only/build/runtimes/runtimes-bins/offload/liboffload/API && /home/botworker/bbot/amdgpu-offload-build-only/build/bin/clang-format -i generated/OffloadEntryPoints.inc.gen && /usr/local/therock-tools/bin/cmake -E copy_if_different generated/OffloadEntryPoints.inc.gen /home/botworker/bbot/amdgpu-offload-build-only/build/runtimes/runtimes-bins/offload/liboffload/API/OffloadEntryPoints.inc
/bin/sh: /home/botworker/bbot/amdgpu-offload-build-only/build/bin/clang-format: Permission denied
[284/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_csupport.cpp.o
[285/1107] Generating OffloadFuncs.inc
FAILED: offload/liboffload/API/OffloadFuncs.inc /home/botworker/bbot/amdgpu-offload-build-only/build/runtimes/runtimes-bins/offload/liboffload/API/OffloadFuncs.inc 
cd /home/botworker/bbot/amdgpu-offload-build-only/build/runtimes/runtimes-bins/offload/liboffload/API && /home/botworker/bbot/amdgpu-offload-build-only/build/bin/clang-format -i generated/OffloadFuncs.inc.gen && /usr/local/therock-tools/bin/cmake -E copy_if_different generated/OffloadFuncs.inc.gen /home/botworker/bbot/amdgpu-offload-build-only/build/runtimes/runtimes-bins/offload/liboffload/API/OffloadFuncs.inc
/bin/sh: /home/botworker/bbot/amdgpu-offload-build-only/build/bin/clang-format: Permission denied
[286/1107] Building CXX object compiler-rt/lib/hwasan/CMakeFiles/RTHwasanAliases.x86_64.dir/hwasan_interceptors.cpp.o
[287/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_sched.cpp.o
[288/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_cancel.cpp.o
[289/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_lock.cpp.o
[290/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_collapse.cpp.o
[291/1107] Building CXX object compiler-rt/lib/dfsan/CMakeFiles/clang_rt.dfsan-x86_64.dir/dfsan.cpp.o
[292/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_version.cpp.o
[293/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/ompd-specific.cpp.o
[294/1107] Building Fortran object flang-rt/lib/runtime/CMakeFiles/flang-rt-mod.dir/iso_fortran_env_impl.f90.o
[295/1107] Building Fortran object flang-rt/lib/runtime/CMakeFiles/flang-rt-mod.dir/__fortran_builtins.f90.o
[296/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_ftn_cdecl.cpp.o
[297/1107] Building Fortran object flang-rt/lib/runtime/CMakeFiles/flang-rt-mod.dir/ieee_features.f90.o
[298/1107] Building CXX object compiler-rt/lib/dfsan/CMakeFiles/clang_rt.dfsan-x86_64.dir/dfsan_allocator.cpp.o
[299/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_atomic.cpp.o
[300/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_tasking.cpp.o
[301/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_ftn_extra.cpp.o
[302/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_taskdeps.cpp.o
[303/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/z_Linux_util.cpp.o
/home/botworker/bbot/amdgpu-offload-build-only/llvm-project/openmp/runtime/src/z_Linux_util.cpp:444:7: warning: unused variable 'status' [-Wunused-variable]
  444 |   int status;
      |       ^~~~~~
/home/botworker/bbot/amdgpu-offload-build-only/llvm-project/openmp/runtime/src/z_Linux_util.cpp:529:7: warning: unused variable 'status' [-Wunused-variable]
  529 |   int status, old_type, old_state;
      |       ^~~~~~
/home/botworker/bbot/amdgpu-offload-build-only/llvm-project/openmp/runtime/src/z_Linux_util.cpp:529:15: warning: unused variable 'old_type' [-Wunused-variable]
  529 |   int status, old_type, old_state;
      |               ^~~~~~~~
Step 7 (build cmake config) failure: build cmake config (failure)
...
[273/1107] Building generated/OffloadAPI.h.gen...
[274/1107] Building generated/OffloadEntryPoints.inc.gen...
[275/1107] Building generated/OffloadFuncs.inc.gen...
[276/1107] Building generated/OffloadImplFuncDecls.inc.gen...
[277/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_wait_release.cpp.o
[278/1107] Building generated/OffloadPrint.hpp.gen...
[279/1107] Building include/OffloadErrcodes.inc...
[280/1107] Building CXX object compiler-rt/lib/dfsan/CMakeFiles/clang_rt.dfsan-x86_64.dir/dfsan_chained_origin_depot.cpp.o
[281/1107] Building include/OffloadInfo.inc...
[282/1107] Generating OffloadAPI.h
FAILED: offload/liboffload/API/OffloadAPI.h /home/botworker/bbot/amdgpu-offload-build-only/build/runtimes/runtimes-bins/offload/liboffload/API/OffloadAPI.h 
cd /home/botworker/bbot/amdgpu-offload-build-only/build/runtimes/runtimes-bins/offload/liboffload/API && /home/botworker/bbot/amdgpu-offload-build-only/build/bin/clang-format -i generated/OffloadAPI.h.gen && /usr/local/therock-tools/bin/cmake -E copy_if_different generated/OffloadAPI.h.gen /home/botworker/bbot/amdgpu-offload-build-only/build/runtimes/runtimes-bins/offload/liboffload/API/OffloadAPI.h
/bin/sh: /home/botworker/bbot/amdgpu-offload-build-only/build/bin/clang-format: Permission denied
[283/1107] Generating OffloadEntryPoints.inc
FAILED: offload/liboffload/API/OffloadEntryPoints.inc /home/botworker/bbot/amdgpu-offload-build-only/build/runtimes/runtimes-bins/offload/liboffload/API/OffloadEntryPoints.inc 
cd /home/botworker/bbot/amdgpu-offload-build-only/build/runtimes/runtimes-bins/offload/liboffload/API && /home/botworker/bbot/amdgpu-offload-build-only/build/bin/clang-format -i generated/OffloadEntryPoints.inc.gen && /usr/local/therock-tools/bin/cmake -E copy_if_different generated/OffloadEntryPoints.inc.gen /home/botworker/bbot/amdgpu-offload-build-only/build/runtimes/runtimes-bins/offload/liboffload/API/OffloadEntryPoints.inc
/bin/sh: /home/botworker/bbot/amdgpu-offload-build-only/build/bin/clang-format: Permission denied
[284/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_csupport.cpp.o
[285/1107] Generating OffloadFuncs.inc
FAILED: offload/liboffload/API/OffloadFuncs.inc /home/botworker/bbot/amdgpu-offload-build-only/build/runtimes/runtimes-bins/offload/liboffload/API/OffloadFuncs.inc 
cd /home/botworker/bbot/amdgpu-offload-build-only/build/runtimes/runtimes-bins/offload/liboffload/API && /home/botworker/bbot/amdgpu-offload-build-only/build/bin/clang-format -i generated/OffloadFuncs.inc.gen && /usr/local/therock-tools/bin/cmake -E copy_if_different generated/OffloadFuncs.inc.gen /home/botworker/bbot/amdgpu-offload-build-only/build/runtimes/runtimes-bins/offload/liboffload/API/OffloadFuncs.inc
/bin/sh: /home/botworker/bbot/amdgpu-offload-build-only/build/bin/clang-format: Permission denied
[286/1107] Building CXX object compiler-rt/lib/hwasan/CMakeFiles/RTHwasanAliases.x86_64.dir/hwasan_interceptors.cpp.o
[287/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_sched.cpp.o
[288/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_cancel.cpp.o
[289/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_lock.cpp.o
[290/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_collapse.cpp.o
[291/1107] Building CXX object compiler-rt/lib/dfsan/CMakeFiles/clang_rt.dfsan-x86_64.dir/dfsan.cpp.o
[292/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_version.cpp.o
[293/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/ompd-specific.cpp.o
[294/1107] Building Fortran object flang-rt/lib/runtime/CMakeFiles/flang-rt-mod.dir/iso_fortran_env_impl.f90.o
[295/1107] Building Fortran object flang-rt/lib/runtime/CMakeFiles/flang-rt-mod.dir/__fortran_builtins.f90.o
[296/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_ftn_cdecl.cpp.o
[297/1107] Building Fortran object flang-rt/lib/runtime/CMakeFiles/flang-rt-mod.dir/ieee_features.f90.o
[298/1107] Building CXX object compiler-rt/lib/dfsan/CMakeFiles/clang_rt.dfsan-x86_64.dir/dfsan_allocator.cpp.o
[299/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_atomic.cpp.o
[300/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_tasking.cpp.o
[301/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_ftn_extra.cpp.o
[302/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/kmp_taskdeps.cpp.o
[303/1107] Building CXX object openmp/runtime/src/CMakeFiles/obj.omp.dir/z_Linux_util.cpp.o
/home/botworker/bbot/amdgpu-offload-build-only/llvm-project/openmp/runtime/src/z_Linux_util.cpp:444:7: warning: unused variable 'status' [-Wunused-variable]
  444 |   int status;
      |       ^~~~~~
/home/botworker/bbot/amdgpu-offload-build-only/llvm-project/openmp/runtime/src/z_Linux_util.cpp:529:7: warning: unused variable 'status' [-Wunused-variable]
  529 |   int status, old_type, old_state;
      |       ^~~~~~
/home/botworker/bbot/amdgpu-offload-build-only/llvm-project/openmp/runtime/src/z_Linux_util.cpp:529:15: warning: unused variable 'old_type' [-Wunused-variable]
  529 |   int status, old_type, old_state;
      |               ^~~~~~~~

```

</details>

https://github.com/llvm/llvm-project/pull/213942