PowerPC64 power7 strncpy stack handling and cfi

This patch moves the frame setup and teardown to immediately around
the single memset call, as has been done for power8.  I've also
decreased FRAMESIZE to that needed to save the two callee-saved
registers used.  Plus added cfi.

	* sysdeps/powerpc/powerpc64/power7/strncpy.S: Decrease FRAMESIZE.
	Move LR save and frame setup/teardown and LR restore to
	immediately around memset call.  Provide cfi.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.vnet.ibm.com>