powerpc: Add a POWER8-optimized version of cosf()

powerpc: Add a POWER8-optimized version of cosf()

This implementation is based on the one already used at
sysdeps/powerpc/powerpc64/fpu/multiarch/s_sinf-power8.S.

	* sysdeps/powerpc/powerpc64/fpu/multiarch/Makefile
	[$(subdir) = math] (libm-sysdep_routines): Add s_cosf-power8 and
	s_cosf-ppc64.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_cosf-power8.S: New file.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_cosf-ppc64.c: Likewise.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_cosf.c: Likewise.
	* sysdeps/powerpc/powerpc64/power8/fpu/s_cosf.S: Likewise.