x86-64: Add log2f with FMA

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

                           Before            After     Improvement
reciprocal-throughput      16.5937          14.0789       17%
latency                    41.7755          35.3586       18%

	* sysdeps/x86_64/fpu/multiarch/Makefile (libm-sysdep_routines):
	Add e_log2f-fma.
	(CFLAGS-e_log2f-fma.c): New.
	* sysdeps/x86_64/fpu/multiarch/e_log2f-fma.c: New file.
	* sysdeps/x86_64/fpu/multiarch/e_log2f.c: Likewise.