Add fall-through comment to i386-tdep.c

Add fall-through comment to i386-tdep.c

This adds a fall-through comment in i386-tdep.c.  I was not sure what
to do here, so I elected to preserve the status quo.  In review, John
Baldwin pointed out that: "I believe this is correct based on the diff
that added the special cases for xgetbv and xsetbv as previously ldgt
and lidt were treated the same".

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

	* i386-tdep.c (i386_process_record): Add fall-through comment.