S390: Optimize strncpy and wcsncpy.

S390: Optimize strncpy and wcsncpy.

This patch provides optimized versions of strncpy and wcsncpy with the z13
vector instructions.

ChangeLog:

	* sysdeps/s390/multiarch/strncpy-vx.S: New File.
	* sysdeps/s390/multiarch/strncpy.c: Likewise.
	* sysdeps/s390/multiarch/wcsncpy-c.c: Likewise.
	* sysdeps/s390/multiarch/wcsncpy-vx.S: Likewise.
	* sysdeps/s390/multiarch/wcsncpy.c: Likewise.
	* sysdeps/s390/s390-32/multiarch/strncpy.c: Likewise.
	* sysdeps/s390/s390-64/multiarch/strncpy.c: Likewise.
	* sysdeps/s390/multiarch/Makefile (sysdep_routines): Add strncpy and
	wcsncpy functions.
	* wcsmbs/wcsncpy.c: Use WCSNCPY if defined.
	* sysdeps/s390/multiarch/ifunc-impl-list.c
	(__libc_ifunc_impl_list): Add ifunc test for strncpy, wcsncpy.
	* string/test-strncpy.c: Add wcsncpy support.
	* wcsmbs/test-wcsncpy.c: New File.
	* wcsmbs/Makefile (strop-tests): Add wcsncpy.
	* benchtests/bench-strncpy.c: Add wcsncpy support.
	* benchtests/bench-wcsncpy.c: New File.
	* benchtests/Makefile (wcsmbs-bench): Add wcsncpy