S390: Refactor memccpy ifunc handling.

S390: Refactor memccpy ifunc handling.

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