S390: Refactor stpcpy ifunc handling.

S390: Refactor stpcpy ifunc handling.

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