S390: Refactor strncpy ifunc handling.

S390: Refactor strncpy ifunc handling.

The ifunc handling for strncpy 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.

Note: The fallback s390-32/s390-64 ifunc variants are now moved to
the strncpy-z900.S files. The s390-32/s390-64 files multiarch/strncpy.c
and strncpy.S are deleted.

ChangeLog:

	* sysdeps/s390/multiarch/Makefile
	(sysdep_routines): Remove strncpy variants.
	* sysdeps/s390/Makefile (sysdep_routines): Add strncpy variants.
	* sysdeps/s390/multiarch/ifunc-impl-list.c
	(__libc_ifunc_impl_list): Refactor ifunc handling for strncpy.
	* sysdeps/s390/multiarch/strncpy-vx.S: Move to ...
	* sysdeps/s390/strncpy-vx.S: ... here and adjust ifunc handling.
	* sysdeps/s390/multiarch/strncpy.c: Move to ...
	* sysdeps/s390/strncpy.c: ... here and adjust ifunc handling.
	* sysdeps/s390/ifunc-strncpy.h: New file.
	* sysdeps/s390/s390-64/strncpy.S: Move to ...
	* sysdeps/s390/s390-64/strncpy-z900.S: ... here
	and adjust ifunc handling.
	* sysdeps/s390/s390-32/strncpy.S: Move to ...
	* sysdeps/s390/s390-32/strncpy-z900.S: ... here
	and adjust ifunc handling.
	* sysdeps/s390/s390-32/multiarch/strncpy.c: Delete file.
	* sysdeps/s390/s390-64/multiarch/strncpy.c: Likewise.