S390: Refactor strncmp ifunc handling.

S390: Refactor strncmp ifunc handling.

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