S390: Optimize strpbrk and wcspbrk.

This patch provides optimized versions of strpbrk and wcspbrk with the z13
vector instructions.

ChangeLog:

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