S390: Refactor memchr ifunc handling.

S390: Refactor memchr ifunc handling.

The ifunc handling for memchr 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.
Glibc internal calls will then also use the "newer" ifunc variant.

Note: The fallback s390-32/s390-64 ifunc variants with srst instruction
are now moved to the unified memchr-z900.S file which can be used for
31/64bit. The s390-32/s390-64 files multiarch/memchr.c and memchr.S
are deleted.

ChangeLog:

	* sysdeps/s390/multiarch/Makefile
	(sysdep_routines): Remove memchr variants.
	* sysdeps/s390/Makefile (sysdep_routines): Add memchr variants.
	* sysdeps/s390/multiarch/ifunc-impl-list.c
	(__libc_ifunc_impl_list): Refactor ifunc handling for memchr.
	* sysdeps/s390/multiarch/memchr-vx.S: Move to ...
	* sysdeps/s390/memchr-vx.S: ... here and adjust ifunc handling.
	* sysdeps/s390/multiarch/memchr.c: Move to ...
	* sysdeps/s390/memchr.c: ... here and adjust ifunc handling.
	* sysdeps/s390/ifunc-memchr.h: New file.
	* sysdeps/s390/s390-64/memchr.S: Move to ...
	* sysdeps/s390/memchr-z900.S: ... here and adjust to be usable
	for 31/64bit and ifunc handling.
	* sysdeps/s390/s390-32/multiarch/memchr.c: Delete file.
	* sysdeps/s390/s390-64/multiarch/memchr.c: Likewise.
	* sysdeps/s390/s390-32/memchr.S: Likewise.