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