powerpc: strstr optimization

This patch optimizes strstr function for power >= 7 systems.  Performance
gain is obtained using aligned memory access and usage of cmpb
instruction for quicker comparison.  The average improvement of this
optimization is ~40%.  Tested on ppc64 and ppc64le.

2015-07-16  Rajalakshmi Srinivasaraghavan  <raji@linux.vnet.ibm.com>

	* sysdeps/powerpc/powerpc64/multiarch/Makefile: Add strstr().
	* sysdeps/powerpc/powerpc64/multiarch/ifunc-impl-list.c: Likewise.
	* sysdeps/powerpc/powerpc64/power7/strstr.S: New File.
	* sysdeps/powerpc/powerpc64/multiarch/strstr-power7.S: New File.
	* sysdeps/powerpc/powerpc64/multiarch/strstr-ppc64.c: New File.
	* sysdeps/powerpc/powerpc64/multiarch/strstr.c: New File.