[clang] [flang] [mlir] [flang] Add basic -mtune support (PR #95043)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `flang-aarch64-libcxx` running on `linaro-flang-aarch64-libcxx` while building `clang,flang,mlir` at step 6 "test-build-unified-tree-check-flang".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/89/builds/791

Here is the relevant piece of the build log for the reference:
```
Step 6 (test-build-unified-tree-check-flang) failure: test (failure)
******************** TEST 'Flang :: Lower/tune-cpu-llvm.f90' FAILED ********************
Exit Code: 2

Command Output (stderr):
--
RUN: at line 1: /home/tcwg-buildbot/worker/flang-aarch64-libcxx/build/bin/flang-new -mtune=pentium4 -S -emit-llvm /home/tcwg-buildbot/worker/flang-aarch64-libcxx/llvm-project/flang/test/Lower/tune-cpu-llvm.f90 -o - | /home/tcwg-buildbot/worker/flang-aarch64-libcxx/build/bin/FileCheck /home/tcwg-buildbot/worker/flang-aarch64-libcxx/llvm-project/flang/test/Lower/tune-cpu-llvm.f90
+ /home/tcwg-buildbot/worker/flang-aarch64-libcxx/build/bin/flang-new -mtune=pentium4 -S -emit-llvm /home/tcwg-buildbot/worker/flang-aarch64-libcxx/llvm-project/flang/test/Lower/tune-cpu-llvm.f90 -o -
+ /home/tcwg-buildbot/worker/flang-aarch64-libcxx/build/bin/FileCheck /home/tcwg-buildbot/worker/flang-aarch64-libcxx/llvm-project/flang/test/Lower/tune-cpu-llvm.f90
flang-new: error: unsupported argument 'pentium4' to option '-mtune='
FileCheck error: '<stdin>' is empty.
FileCheck command line:  /home/tcwg-buildbot/worker/flang-aarch64-libcxx/build/bin/FileCheck /home/tcwg-buildbot/worker/flang-aarch64-libcxx/llvm-project/flang/test/Lower/tune-cpu-llvm.f90

--

********************


```

https://github.com/llvm/llvm-project/pull/95043