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.

sysdeps/s390/strstr.c

@@ -0,0 +1,40 @@
+/* Multiple versions of strstr.
+   Copyright (C) 2018 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <ifunc-strstr.h>
+
+#if HAVE_STRSTR_IFUNC
+# define strstr __redirect_strstr
+# include <string.h>
+# include <ifunc-resolve.h>
+# undef strstr
+
+# if HAVE_STRSTR_C
+extern __typeof (__redirect_strstr) STRSTR_C attribute_hidden;
+# endif
+
+# if HAVE_STRSTR_Z13
+extern __typeof (__redirect_strstr) STRSTR_Z13 attribute_hidden;
+# endif
+
+s390_libc_ifunc_expr (__redirect_strstr, strstr,
+		      (HAVE_STRSTR_Z13 && (hwcap & HWCAP_S390_VX))
+		      ? STRSTR_Z13
+		      : STRSTR_DEFAULT
+		      )
+#endif