math: Ensure HUGE_VAL, HUGE_VALF, HUGE_VALL are defined.

math: Ensure HUGE_VAL, HUGE_VALF, HUGE_VALL are defined.

* lib/math.in.h (HUGE_VAL, HUGE_VALF, HUGE_VALL): Define fallbacks.
* tests/test-math.c: Include macros.h. Check that HUGE_VAL, HUGE_VALF,
HUGE_VALL are defined.
(numeric_equald): Renamed from numeric_equal.
(numeric_equalf, numeric_equall): New functions.
(main): Check also HUGE_VALF, HUGE_VALL.
* modules/math-tests (Files): Add tests/macros.h.
* doc/posix-headers/math.texi: Document the problems with HUGE_VALF and
HUGE_VALL.