[llvm] Export empty vt_gen target (for standalone builds) (PR #109817)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-s390x-linux` running on `systemz-1` while building `llvm` at step 4 "build stage 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/42/builds/1190

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

```
Step 4 (build stage 1) failure: 'ninja -j4' (failure)
...
[1819/5822] Building CXX object lib/ExecutionEngine/Orc/CMakeFiles/LLVMOrcJIT.dir/SpeculateAnalyses.cpp.o
[1820/5822] Building CXX object lib/ExecutionEngine/Orc/Debugging/CMakeFiles/LLVMOrcDebugging.dir/DebugInfoSupport.cpp.o
[1821/5822] Building CXX object lib/ExecutionEngine/Orc/Debugging/CMakeFiles/LLVMOrcDebugging.dir/DebuggerSupport.cpp.o
[1822/5822] Building CXX object lib/ExecutionEngine/Orc/Debugging/CMakeFiles/LLVMOrcDebugging.dir/LLJITUtilsCBindings.cpp.o
[1823/5822] Building CXX object lib/ExecutionEngine/Orc/Debugging/CMakeFiles/LLVMOrcDebugging.dir/DebuggerSupportPlugin.cpp.o
[1824/5822] Building CXX object lib/ExecutionEngine/Orc/TargetProcess/CMakeFiles/LLVMOrcTargetProcess.dir/JITLoaderGDB.cpp.o
[1825/5822] Building CXX object lib/ExecutionEngine/Orc/Debugging/CMakeFiles/LLVMOrcDebugging.dir/PerfSupportPlugin.cpp.o
[1826/5822] Building CXX object lib/ExecutionEngine/Orc/Debugging/CMakeFiles/LLVMOrcDebugging.dir/VTuneSupportPlugin.cpp.o
[1827/5822] Building CXX object lib/ExecutionEngine/Orc/TargetProcess/CMakeFiles/LLVMOrcTargetProcess.dir/ExecutorSharedMemoryMapperService.cpp.o
[1828/5822] Building CXX object lib/ExecutionEngine/Orc/TargetProcess/CMakeFiles/LLVMOrcTargetProcess.dir/RegisterEHFrames.cpp.o
FAILED: lib/ExecutionEngine/Orc/TargetProcess/CMakeFiles/LLVMOrcTargetProcess.dir/RegisterEHFrames.cpp.o 
CCACHE_CPP2=yes CCACHE_HASHDIR=yes /usr/bin/ccache /usr/bin/c++ -DGTEST_HAS_RTTI=0 -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/uweigand/sandbox/buildbot/clang-s390x-linux/stage1/lib/ExecutionEngine/Orc/TargetProcess -I/home/uweigand/sandbox/buildbot/clang-s390x-linux/llvm/llvm/lib/ExecutionEngine/Orc/TargetProcess -I/home/uweigand/sandbox/buildbot/clang-s390x-linux/stage1/include -I/home/uweigand/sandbox/buildbot/clang-s390x-linux/llvm/llvm/include -fPIC -fvisibility-inlines-hidden -Werror=date-time -fno-lifetime-dse -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wno-missing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough -Wno-uninitialized -Wno-nonnull -Wno-class-memaccess -Wno-redundant-move -Wno-pessimizing-move -Wno-noexcept-type -Wdelete-non-virtual-dtor -Wsuggest-override -Wno-comment -Wno-misleading-indentation -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG  -fno-exceptions -funwind-tables -fno-rtti -UNDEBUG -std=c++17 -MD -MT lib/ExecutionEngine/Orc/TargetProcess/CMakeFiles/LLVMOrcTargetProcess.dir/RegisterEHFrames.cpp.o -MF lib/ExecutionEngine/Orc/TargetProcess/CMakeFiles/LLVMOrcTargetProcess.dir/RegisterEHFrames.cpp.o.d -o lib/ExecutionEngine/Orc/TargetProcess/CMakeFiles/LLVMOrcTargetProcess.dir/RegisterEHFrames.cpp.o -c /home/uweigand/sandbox/buildbot/clang-s390x-linux/llvm/llvm/lib/ExecutionEngine/Orc/TargetProcess/RegisterEHFrames.cpp
ccache: error: execv of /usr/bin/c++ failed: No such file or directory
[1829/5822] Building CXX object lib/ExecutionEngine/Orc/TargetProcess/CMakeFiles/LLVMOrcTargetProcess.dir/JITLoaderVTune.cpp.o
[1830/5822] Building CXX object lib/ExecutionEngine/Orc/TargetProcess/CMakeFiles/LLVMOrcTargetProcess.dir/JITLoaderPerf.cpp.o
[1831/5822] Building CXX object lib/ExecutionEngine/Orc/TargetProcess/CMakeFiles/LLVMOrcTargetProcess.dir/OrcRTBootstrap.cpp.o
ninja: build stopped: subcommand failed.

```

</details>

https://github.com/llvm/llvm-project/pull/109817