x86-64: Check Prefer_FSRM in ifunc-memmove.h

Although the REP MOVSB implementations of memmove, memcpy and mempcpy
aren't used by the current processors, this patch adds Prefer_FSRM
check in ifunc-memmove.h so that they can be used in the future.

	* sysdeps/x86/cpu-features.h (bit_arch_Prefer_FSRM): New.
	(index_arch_Prefer_FSRM): Likewise.
	* sysdeps/x86/cpu-tunables.c (TUNABLE_CALLBACK (set_hwcaps)):
	Also check Prefer_FSRM.
	* sysdeps/x86_64/multiarch/ifunc-memmove.h (IFUNC_SELECTOR):
	Also return OPTIMIZE (erms) for Prefer_FSRM.