S390: Optimize memrchr.

This patch provides optimized version of memrchr with the z13 vector
instructions.

ChangeLog:

	* sysdeps/s390/multiarch/memrchr-c.c: New File.
	* sysdeps/s390/multiarch/memrchr-vx.S: Likewise.
	* sysdeps/s390/multiarch/memrchr.c: Likewise.
	* sysdeps/s390/multiarch/Makefile
	(sysdep_routines): Add memrchr functions.
	* sysdeps/s390/multiarch/ifunc-impl-list-common.c
	(__libc_ifunc_impl_list_common): Add ifunc test for memrchr.