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

[Louis Dionne via cfe-commits]

ldionne wrote:

@rorth Please check if this is still broken after https://github.com/llvm/llvm-project/commit/7ab643383fe49d5bb22e45610cb52476bf7a922e

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