S390: Refactor memcpy/mempcpy ifunc handling.

S390: Refactor memcpy/mempcpy ifunc handling.

This patch moves all ifunc variants for memcpy/mempcpy
to sysdeps/s390/memcpy-z900.S. The configure-check/preprocessor logic
in sysdeps/s390/ifunc-memcpy.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-memcpy.h: New File.
	* sysdeps/s390/memcpy.S: Move to ...
	* sysdeps/s390/memcpy-z900.S ... here.
	Move implementations from memcpy-s390x.s to here.
	* sysdeps/s390/multiarch/memcpy-s390x.S: Delete File.
	* sysdeps/s390/multiarch/Makefile (sysdep_routines):
	Remove memcpy/mempcpy variants.
	* sysdeps/s390/Makefile (sysdep_routines):
	Add memcpy/mempcpy variants.
	* sysdeps/s390/multiarch/ifunc-impl-list.c
	(__libc_ifunc_impl_list): Adjust ifunc variants for
	memcpy and mempcpy.
	* sysdeps/s390/multiarch/memcpy.c: Move ifunc resolver
	to ...
	* sysdeps/s390/memcpy.c: ... here.
	Adjust ifunc variants for memcpy.
	* sysdeps/s390/multiarch/mempcpy.c: Move to ...
	* sysdeps/s390/mempcpy.c: ... here.
	Adjust ifunc variants for mempcpy.
	* sysdeps/s390/mempcpy.S: Delete file.