S390: Optimize memccpy.

This patch provides optimized versions of memccpy with the z13 vector
instructions.

ChangeLog:

	* sysdeps/s390/multiarch/memccpy-c.c: New File.
	* sysdeps/s390/multiarch/memccpy-vx.S: Likewise.
	* sysdeps/s390/multiarch/memccpy.c: Likewise.
	* sysdeps/s390/multiarch/Makefile
	(sysdep_routines): Add memccpy functions.
	* sysdeps/s390/multiarch/ifunc-impl-list-common.c
	(__libc_ifunc_impl_list_common): Add ifunc test for memccpy.
	* string/memccpy.c: Use MEMCCPY if defined.