Show padding in ptype/o output

Show padding in ptype/o output

I was recently using ptype/o to look at the layout of some objects in
gdb.  I noticed that trailing padding was not shown -- but I wanted to
be able to look at that, too.

This patch changes ptype/o to also print trailing holes.

Tested on x86-64 Fedora 26.

gdb/ChangeLog
2018-05-18  Tom Tromey  <tom@tromey.com>

	* c-typeprint.c (maybe_print_hole): New function.
	(c_print_type_struct_field_offset): Update.
	(c_type_print_base_struct_union): Call maybe_print_hole.

gdb/testsuite/ChangeLog
2018-05-18  Tom Tromey  <tom@tromey.com>

	* gdb.base/ptype-offsets.exp: Update.