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

[Nikita Popov via llvm-commits]

nikic wrote:

Ping

To provide a bit of extra context here, this patch is not needed when the LLVM assertions build sets these macros (using `-D`), this problem shows up in our distro builds where these are injected via CFLAGS with `-Wp,-D`.

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