[openmp][cmake] Match GPU triples consistently in openmp/module (#211138)

Fixes #211135.

`openmp/module/CMakeLists.txt:29` gates the GPU-only Fortran compile
options on `"^amdgcn|^nvptx"`, while `openmp/CMakeLists.txt:176-177`
selects host-vs-device layout for the same build using
`"^amdgpu|^amdgcn|^nvptx|^spirv64"` against both
`LLVM_DEFAULT_TARGET_TRIPLE` and `CMAKE_CXX_COMPILER_TARGET`.

All four offload cache files use the triple `amdgpu-amd-amdhsa`, which
`^amdgcn` does not match, so `-nogpulib -flto` are silently not applied
to `libomp-mod` in the recommended AMDGPU offload configurations.

Compute the test once as `LIBOMP_TARGET_IS_GPU` in
`openmp/CMakeLists.txt` and use it at both sites, rather than
duplicating a widened regex. The conditions differ, not just the
patterns: copying the regex alone would leave a build that sets
`CMAKE_CXX_COMPILER_TARGET` without `LLVM_DEFAULT_TARGET_TRIPLE` still
taking the device path in `openmp/` while `libomp-mod` misses the flags.
`libomp-mod` compiles Fortran, so `CMAKE_Fortran_COMPILER_TARGET` is
also consulted; neither site did before.

Evaluated with CMake's regex engine:

| triple | before | after |
|---|---|---|
| `amdgpu-amd-amdhsa` | 0 | 1 |
| `amdgcn-amd-amdhsa` | 1 | 1 |
| `nvptx64-nvidia-cuda` | 1 | 1 |
| `spirv64-amd-amdhsa` | 0 | 1 |
| `x86_64-unknown-linux-gnu` | 0 | 0 |

`^spirv64` only brings `module/` in line with
`openmp/CMakeLists.txt:176`, which already routes spirv64 to `device/`.
Host builds are unaffected.

Roughly five other sites carry the same `amdgcn`-only pattern
(`offload/CMakeLists.txt:30`, `flang-rt/CMakeLists.txt:126`,
`flang-rt/lib/runtime/CMakeLists.txt:317`,
`flang-rt/cmake/modules/AddFlangRT.cmake:298`,
`cmake/Modules/GetToolchainDirs.cmake:118`) and are left for a
follow-up.

No test: configure-time logic, no test mechanism exists in `openmp/`.

This affects performance-critical applications on large AMD GPU
supercomputers, including [MFC](https://github.com/MFlowCode/MFC).

All numbers above come from the validated reproducers included with this
report and are independently reproducible; they stand on their own.

This was found and root-caused with the assistance of AI tools.

GitOrigin-RevId: 2f5771c8ecc5a9edd4469e08255402436dc7dd31