[llvm] [flang-rt] Use -Wp to undefine macros (PR #156034)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

`FLANG_RT_SUPPORTS_UNDEFINE_FLAG` checks whether `-U` is supported (e.g. in msvc it would be `/U`, but GLIBCXX/LIBCPP does not support those). I think
https://github.com/llvm/llvm-project/blob/c09cc2c5a3d57506d8744fa889f35a6aa260a52b/flang-rt/CMakeLists.txt#L262
should be updated to test for `-Wp,-U`

https://github.com/llvm/llvm-project/pull/156034