S390: Optimize strcspn and wcscspn.

This patch provides optimized versions of strcspn and wcscspn with the z13
vector instructions.

ChangeLog:

	* sysdeps/s390/multiarch/strcspn-c.c: New File.
	* sysdeps/s390/multiarch/strcspn-vx.S: Likewise.
	* sysdeps/s390/multiarch/strcspn.c: Likewise.
	* sysdeps/s390/multiarch/wcscspn-c.c: Likewise.
	* sysdeps/s390/multiarch/wcscspn-vx.S: Likewise.
	* sysdeps/s390/multiarch/wcscspn.c: Likewise.
	* sysdeps/s390/multiarch/Makefile (sysdep_routines): Add strcspn and
	wcscspn functions.
	* sysdeps/s390/multiarch/ifunc-impl-list.c
	(__libc_ifunc_impl_list): Add ifunc test for strcspn, wcscspn.
	* wcsmbs/wcscspn.c: Use WCSCSPN if defined.
	* string/test-strcspn.c: Add wcscspn support.
	* wcsmbs/test-wcscspn.c: New File.
	* wcsmbs/Makefile (strop-tests): Add wcscspn.
	* benchtests/bench-strcspn.c: Add wcscspn support.
	* benchtests/bench-wcscspn.c: New File.
	* benchtests/Makefile (wcsmbs-bench): Add wcscspn.