S390: Refactor strcat ifunc handling.

S390: Refactor strcat ifunc handling.

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