[libc++] Assume that builtins for math.h functions are available

All supported compilers support the builtins, so this removes dead code and simplifies a lot of the functions through that.

Reviewed By: ldionne, #libc, EricWF

Spies: EricWF, michaelplatings, libcxx-commits

Differential Revision: https://reviews.llvm.org/D135781