Add two fall-through comments in rs6000-tdep.c

Add two fall-through comments in rs6000-tdep.c

This adds two fall-through comments in rs6000-tdep.c.  I looked at the
PPC instruction manual and convinced myself that this was correct.
And, this isn't a semantic change.  However, close review would still
be good.

2018-05-04  Tom Tromey  <tom@tromey.com>

	* rs6000-tdep.c (ppc_process_record_op4)
	(ppc_process_record_op63): Add fall-through comment.