x86-64: Add powf with FMA

For workload-spec2017.wrf, on Skylake, it improves performance by:

                           Before            After     Improvement
reciprocal-throughput      35.4713          27.3842       29%
latency                    82.4537          66.3175       24%

	* sysdeps/x86_64/fpu/multiarch/Makefile (libm-sysdep_routines):
	Add e_powf-fma.
	(CFLAGS-e_powf-fma.c): New.
	* sysdeps/x86_64/fpu/multiarch/e_powf-fma.c: New file.
	* sysdeps/x86_64/fpu/multiarch/e_powf.c: Likewise.