S390: Refactor strpbrk ifunc handling.

S390: Refactor strpbrk ifunc handling.

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