[GOLD] correct grouping of stubs

[GOLD] correct grouping of stubs

This patch rewrites the rather obscure can_add_to_stub_group, fixing
a problem with the handling of sections containing conditional
external branches.  When a section group contains any such section,
the group size needs to be limited to a much smaller size than groups
with only non-conditional external branches.

	PR 20523
	* powerpc.cc (class Stub_control): Add has14_.  Comment owner_.
	(Stub_control::can_add_to_stub_group): Correct grouping of
	sections containing 14-bit external branches.  When returning
	false, set state_ to reflect the fact that we have one section
	for the next group.  Rewrite most of function for clarity.
	Add and expand comments.
	(Target_powerpc::do_relax): Print stub group size retry in hex.