S390: Refactor strncat ifunc handling.

S390: Refactor strncat ifunc handling.

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