PowerPC LE strnlen

PowerPC LE strnlen
http://sourceware.org/ml/libc-alpha/2013-08/msg00098.html

The existing strnlen code has a number of defects, so this patch is more
than just adding little-endian support.  The changes here are similar to
those for memchr.

	* sysdeps/powerpc/powerpc64/power7/strnlen.S (strnlen): Add
	little-endian support.  Remove unnecessary "are we done" tests.
	Handle "s" wrapping around zero and extremely large "size".
	Correct main loop count.  Handle single left-over word from main
	loop inline rather than by using small_loop.  Correct comments.
	Delete "zero" tail, use "end_max" instead.
	* sysdeps/powerpc/powerpc32/power7/strnlen.S: Likewise.