S390: Optimize strlen and wcslen.

S390: Optimize strlen and wcslen.

This patch provides optimized versions of strlen and wcslen with the z13 vector
instructions.
The helper macro IFUNC_VX_IMPL is introduced and is used to register all
__<func>_c() and __<func>_vx() functions within __libc_ifunc_impl_list()
to the ifunc test framework.

ChangeLog:

	* sysdeps/s390/multiarch/Makefile: New File.
	* sysdeps/s390/multiarch/strlen-c.c: Likewise.
	* sysdeps/s390/multiarch/strlen-vx.S: Likewise.
	* sysdeps/s390/multiarch/strlen.c: Likewise.
	* sysdeps/s390/multiarch/wcslen-c.c: Likewise.
	* sysdeps/s390/multiarch/wcslen-vx.S: Likewise.
	* sysdeps/s390/multiarch/wcslen.c: Likewise.
	* string/strlen.c (STRLEN): Define and use macro.
	* sysdeps/s390/multiarch/ifunc-impl-list.c
	(IFUNC_VX_IMPL): New macro function.
	(__libc_ifunc_impl_list): Add ifunc test for strlen, wcslen.
	* benchtests/Makefile (wcsmbs-bench): New variable.
	(string-bench-all): Added wcsmbs-bench.
	* benchtests/bench-wcslen.c: New File.