S390: Refactor strspn ifunc handling.

S390: Refactor strspn ifunc handling.

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