[libc] Make our public headers' macro definitions nicer.

Currently many of our headers' macro definitions are just straight up `#define`.
For instance: https://github.com/llvm/llvm-project/blob/main/libc/include/llvm-libc-macros/fenv-macros.h

It would be nicer to have them check whether they are already defined, or provide
some default/backup if needed.