S390: Refactor strrchr ifunc handling.

S390: Refactor strrchr ifunc handling.

The ifunc handling for strrchr 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.

ChangeLog:

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