S390: Refactor strnlen ifunc handling.

S390: Refactor strnlen ifunc handling.

The ifunc handling for strnlen is adjusted in order to omit ifunc
variants if those will never be used as the minimum architecture level
already supports newer CPUs by default.
Glibc internal calls will then also use the "newer" ifunc variant.

ChangeLog:

	* sysdeps/s390/multiarch/Makefile
	(sysdep_routines): Remove strnlen variants.
	* sysdeps/s390/Makefile (sysdep_routines): Add strnlen variants.
	* sysdeps/s390/multiarch/ifunc-impl-list.c
	(__libc_ifunc_impl_list): Refactor ifunc handling for strnlen.
	* sysdeps/s390/multiarch/strnlen-c.c: Move to ...
	* sysdeps/s390/strnlen-c.c: ... here and adjust ifunc handling.
	* sysdeps/s390/multiarch/strnlen-vx.S: Move to ...
	* sysdeps/s390/strnlen-vx.S: ... here and adjust ifunc handling.
	* sysdeps/s390/multiarch/strnlen.c: Move to ...
	* sysdeps/s390/strnlen.c: ... here and adjust ifunc handling.
	* sysdeps/s390/ifunc-strnlen.h: New file.