MIPSr6/math: Use builtin fma and fmaf

MIPSr6 has MADDF.s/MADDF.d instructions, which are fused.

In MIPS ISA, double support can be subsetted.  Only FMAF is enabled
for this case.

	* sysdeps/mips/fpu/math-use-builtins-fma.h

Signed-off-by: YunQiang Su <syq@gcc.gnu.org>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>