S390: Refactor memset ifunc handling.

S390: Refactor memset ifunc handling.

This patch moves all ifunc variants for memset
to sysdeps/s390/memset-z900.S. The configure-check/preprocessor logic
in sysdeps/s390/ifunc-memset.h decides if ifunc is needed at all
and which ifunc variants should be available.
E.g. if the compiler/assembler already supports z196 by default,
the older ifunc variants are not included.
If we only need the newest ifunc variant,
then we can skip ifunc at all.

Therefore the ifunc-resolvers and __libc_ifunc_impl_list are adjusted
in order to handle only the available ifunc variants.

ChangeLog:

	* sysdeps/s390/ifunc-memset.h: New File.
	* sysdeps/s390/memset.S: Move to ...
	* sysdeps/s390/memset-z900.S ... here.
	Move implementations from memset-s390x.s to here.
	* sysdeps/s390/multiarch/memset-s390x.S: Delete File.
	* sysdeps/s390/multiarch/Makefile (sysdep_routines):
	Remove memset variants.
	* sysdeps/s390/Makefile (sysdep_routines):
	Add memset variants.
	* sysdeps/s390/multiarch/ifunc-impl-list.c
	(__libc_ifunc_impl_list): Adjust ifunc variants for
	memset.
	* sysdeps/s390/multiarch/memset.c: Move ifunc resolver
	to ...
	* sysdeps/s390/memset.c: ... here.
	Adjust ifunc variants for memset.