math.h: Remove support for old gcc versions

Remove support for old gcc versions by unconditionally defining
MATH_BUILTIN_RELOPS and MATH_BUILTIN_CONSTANTS. Per kib's request,
don't #undef those so it's easier to understand what the builtins are
doing.

Reviewed by: brooks
Differential Revision: https://reviews.freebsd.org/D45655
Sponsored by: Netflix