S390: Refactor stpncpy ifunc handling.

S390: Refactor stpncpy ifunc handling.

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