S390: Refactor strchr ifunc handling.

S390: Refactor strchr ifunc handling.

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