S390: Refactor strchrnul ifunc handling.

S390: Refactor strchrnul ifunc handling.

The ifunc handling for strchrnul 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.

ChangeLog:

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