PowerPC64 strncpy, stpncpy and strstr fixes

Makes __stpncpy_power8 call __memset_power8 directly rather than via an
IFUNC.  Fixes a missing _mcount, and removes some redundant NOPS.  The
*_is_local defines are also used in a followup patch.

	* sysdeps/powerpc/powerpc64/multiarch/strncpy-power7.S: Define
	MEMSET_is_local.
	* sysdeps/powerpc/powerpc64/multiarch/strncpy-power8.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/stpncpy-power7.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/stpncpy-power8.S: Likewise.
	Define MEMSET.
	* sysdeps/powerpc/powerpc64/multiarch/strstr-power7.S: Define
	STRLEN_is_local, STRNLEN_is_local, and STRCHR_is_local.
	* sysdeps/powerpc/powerpc64/power7/strstr.S: Likewise.  Don't add
	nop after local calls.
	* sysdeps/powerpc/powerpc64/power7/strncpy.S: Define MEMSET_is_local.
	Don't add nop after local call.
	* sysdeps/powerpc/powerpc64/power8/strncpy.S: Likewise.  Add missing
	CALL_MCOUNT.