S390: Add z13 strstr ifunc variant.

S390: Add z13 strstr ifunc variant.

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

ChangeLog:

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