S390: Add arch13 strstr ifunc variant.

This patch introduces the new arch13 ifunc variant for strstr.
For needles longer than 9 charachters it is relying on the common-code
implementation.  For shorter needles it is using the new vstrs instruction
which is able to search a substring within a vector register.

ChangeLog:

	* sysdeps/s390/Makefile (sysdep_routines): Add strstr-arch13.
	* sysdeps/s390/ifunc-strstr.h (HAVE_STRSTR_ARCH13, STRSTR_ARCH13,
	STRSTR_Z13_ONLY_USED_AS_FALLBACK, HAVE_STRSTR_IFUNC_AND_ARCH13_SUPPORT):
	New defines.
	* sysdeps/s390/multiarch/ifunc-impl-list.c
	(__libc_ifunc_impl_list): Add ifunc variant for arch13 strstr.
	* sysdeps/s390/strstr-arch13.S: New file.
	* sysdeps/s390/strstr-vx.c: Omit GI symbol for z13 strstr ifunc variant
	if it is only used as fallback.
	* sysdeps/s390/strstr.c (strstr): Add arch13 variant in ifunc selector.