S390: Add z13 memmem ifunc variant.

S390: Add z13 memmem ifunc variant.

The new vector variant of memmem is using the common code
implementation, but instead of calling the default
mem* functions, the vector variants are called.

ChangeLog:

	* sysdeps/s390/Makefile (sysdep_routines): Add memmem variants.
	* sysdeps/s390/multiarch/ifunc-impl-list.c
	(__libc_ifunc_impl_list): Add ifunc variants for memmem.
	* sysdeps/s390/ifunc-memmem.h: New file.
	* sysdeps/s390/memmem.c: Likewise.
	* sysdeps/s390/memmem-c.c: Likewise.
	* sysdeps/s390/memmem-vx.c: Likewise.