math.h: Assume gcc, gnu c extensions, and modern compiler

[imp]

Summary

Sponsored by: Netflix

Test Plan

It's my belief that we can't compile without a gnu compiler.
I don't think that this will break anything in the ports tree, but will do an exp run.

[imp]

exp run https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=270249

[kib]

lib/msun/src/math.h

122

For me, the !GNU block clearly shows the intent of the macros, while leaving only the built-in version requires use of gcc docs to figure out the contract. The non-GCC version should be left there, even if as a comment, IMO.

[imp]

On second thought, go ahead and drop doing this for now.