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

[Louis Dionne via llvm-commits]

ldionne wrote:

Yeah I think this is a great cleanup, I think we just need to fix the CI issues.

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