[libc++] Use builtins in more math.h functions.

Not using builtins doesn't always imply worse code,
but for e. g. isinf, this is 30%+ faster.

Before:
name        time/op
BM_isinf     2.14ns ± 2%

After:
name        time/op
BM_isinf     1.33ns ± 2%

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D88854