S390: Refactor wcsncmp ifunc handling.

S390: Refactor wcsncmp ifunc handling.

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