[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:

The failing tests here previously were clearly unrelated (missing python module `packaging` somewhere). The failure I see now is due to a timeout (also unrelated):
```
llvm-lit: /home/runner/_work/llvm-project/llvm-project/llvm/utils/lit/lit/main.py:72: note: The test suite configuration requested an individual test timeout of 0 seconds but a timeout of 1500 seconds was requested on the command line. Forcing timeout to be 1500 seconds.
-- Testing: 9661 tests, 30 workers --
  interrupted by user, skipping remaining tests
```

CI hasn't finished yet, so there may be others things hiding still, but I don't consider it realistic (to be caused by the changes here). The code before/after this PR should work exactly as before, we're just stripping the fallbacks for old CMake.

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