powerpc: Optimized strncat for POWER7/PPC64

With 3eb3879 (Simplify strncat) the generic algorithms uses
strlen, strnlen, and memcpy.  This is faster than POWER7 current
implementation, especially for unaligned strings (where POWER7 code
uses byte-byte operations).

This patch removes the assembly implementation and uses a multiarch
specialization based on default algorithm calling optimized POWER7
symbols.

ChangeLog

@@ -1,5 +1,9 @@
 2015-01-13  Adhemerval Zanella  <azanella@linux.vnet.ibm.com>
 
+	* sysdeps/powerpc/powerpc64/multiarch/strncat-power7.c: New file.
+	* sysdeps/powerpc/powerpc64/multiarch/strncat-power7.S: Remove file.
+	* sysdeps/powerpc/powerpc64/power7/strncat.S: Likewise.
+
 	* sysdeps/powerpc/powerpc64/multiarch/Makefile [sysdep_routines]: Add
 	strncat-power8 object.
 	* sysdeps/powerpc/powerpc64/multiarch/strcat.c (strcat): Add

sysdeps/powerpc/powerpc64/multiarch/strncat-power7.c

@@ -0,0 +1,31 @@
+/* Copyright (C) 2015 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 <string.h>
+
+#define STRNCAT __strncat_power7
+
+extern __typeof (strncat) __strncat_power7 attribute_hidden;
+extern __typeof (strlen) __strlen_power7 attribute_hidden;
+extern __typeof (strnlen) __strnlen_power7 attribute_hidden;
+extern __typeof (memcpy) __memcpy_power7 attribute_hidden;
+
+#define strlen    __strlen_power7
+#define __strnlen __strnlen_power7
+#define memcpy    __memcpy_power7
+
+#include <string/strncat.c>