math: Improve fmod(f) performance

Optimize the fast paths (x < y) and (x/y < 2^12).  Delay handling of special
cases to reduce the number of instructions executed before the fast paths.
Performance improvements for fmod:

		Skylake	Zen2	Neoverse V1
subnormals	11.8%	4.2%	11.5%
normal		3.9%	0.01%	-0.5%
close-exponents	6.3%	5.6%	19.4%

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>