[GOLD] PowerPC relaxation corner case

[GOLD] PowerPC relaxation corner case

It's possible for the section grouping code to decide that the last
section looked at (the first section by address) doesn't fit into a
group.  In one case that section is already a group owner and that is
handled correctly.  In other cases the section should be put into its
own group.  Like this:

	* powerpc.cc (Stub_control::set_output_and_owner): New function.
	(Target_powerpc::group_sections): Use it.