S390: Refactor wmemcmp ifunc handling.

The ifunc handling for wmemcmp is adjusted in order to omit ifunc
variants if those will never be used as the minimum architecture level
already supports newer CPUs by default.

ChangeLog:

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