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

[via llvm-commits]

https://github.com/h-vetinari commented:

> Here's a radical question, do we really want to use CMake's support for this?

I think this is a fair question in the big picture, but the current state of the code is _already_ using CMake for that (see below) - I'm just removing the fallbacks for old CMake versions.

IMO this PR should stay a trivial clean-up, and any other flag-check-refactorings should come on top.

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