S390: Refactor wmemset ifunc handling.

S390: Refactor wmemset ifunc handling.

The ifunc handling for wmemset is adjusted in order to omit ifunc
if the minimum architecture level already supports newer CPUs by default.
Unfortunately the c ifunc variant can't be omitted at all as it is used
by the z13 ifunc variant as fallback if the pointers are not 4-byte aligned.
Glibc internal calls will use the "newer" ifunc variant.

ChangeLog:

	* sysdeps/s390/multiarch/Makefile
	(sysdep_routines): Remove wmemset variants.
	* sysdeps/s390/Makefile (sysdep_routines): Add wmemset variants.
	* sysdeps/s390/multiarch/ifunc-impl-list.c
	(__libc_ifunc_impl_list): Refactor ifunc handling for wmemset.
	* sysdeps/s390/multiarch/wmemset-c.c: Move to ...
	* sysdeps/s390/wmemset-c.c: ... here and adjust ifunc handling.
	* sysdeps/s390/multiarch/wmemset-vx.S: Move to ...
	* sysdeps/s390/wmemset-vx.S: ... here and adjust ifunc handling.
	* sysdeps/s390/multiarch/wmemset.c: Move to ...
	* sysdeps/s390/wmemset.c: ... here and adjust ifunc handling.
	* sysdeps/s390/ifunc-wmemset.h: New file.