S390: Optimize strchrnul and wcschrnul.

S390: Optimize strchrnul and wcschrnul.

This patch provides optimized versions of strchrnul and wcschrnul with the z13
vector instructions.

ChangeLog:

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