x86-64: Add exp2f with FMA

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

                           Before            After     Improvement
reciprocal-throughput      13.0291          11.2225       16%
latency                    44.5154          37.5766       18%

	* sysdeps/x86_64/fpu/multiarch/Makefile (libm-sysdep_routines):
	Add e_exp2f-fma.
	(CFLAGS-e_exp2f-fma.c): New.
	* sysdeps/x86_64/fpu/multiarch/e_exp2f-fma.c: New file.
	* sysdeps/x86_64/fpu/multiarch/e_exp2f.c: Likewise.