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

[Ryan Prichard via llvm-commits]

rprichard wrote:

> This check in libcxx/cmake/config-ix.cmake seems to break:
> 
> ```
> check_cxx_compiler_flag(-nostdlib++ CXX_SUPPORTS_NOSTDLIBXX_FLAG)
> ```

This particular check was already using `check_cxx_compiler_flag` before this PR, so it was broken(?), but it apparently doesn't matter because the real check is in runtimes/CMakeLists.txt instead, and that was previously using `llvm_check_compiler_linker_flag`.

Not sure how the runtimes build influences the libcxx/libcxxabi builds. It feels like `CXX_SUPPORTS_NOSTDLIBXX_FLAG` from the runtimes build is being inherited?


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