S390: Refactor wcschrnul ifunc handling.

S390: Refactor wcschrnul ifunc handling.

The ifunc handling for wcschrnul 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.

ChangeLog:

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