x86-64: Add cosf with FMA

On Skylake, bench-cosf reports performance improvement:

            Before        After         Improvement
max        135.362       94.552            43%
min        8.532         7.688             11%
mean       17.1446       11.8128           45%

	* sysdeps/x86_64/fpu/multiarch/Makefile (libm-sysdep_routines):
	Add s_cosf-sse2 and s_cosf-fma.
	(CFLAGS-s_cosf-fma.c): New.
	* sysdeps/x86_64/fpu/multiarch/s_cosf-fma.c: New file.
	* sysdeps/x86_64/fpu/multiarch/s_cosf-sse2.c: Likewise.
	* sysdeps/x86_64/fpu/multiarch/s_cosf.c: Likewise.

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