[clang] [compiler-rt] [libcxx] [libunwind] [llvm] [openmp] [cmake] switch to CMake's native `check_{compiler,linker}_flag` (PR #96171)

[via llvm-commits]

h-vetinari wrote:

Failing bootstrap build still has:
```
    from packaging import version
ModuleNotFoundError: No module named 'packaging'
```
which is clearly unrelated to this PR.

https://github.com/llvm/llvm-project/pull/96171