S390: Optimize wmemcmp.

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

ChangeLog:

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