[GOLD] Further tidy to powerpc can_add_to_stub_group

This patch adds a little more debug output, and replaces two variables
with one, tracking current max group size by group_size_ rather than
by has14_.

	* powerpc.cc (class Stub_control): Delete stub14_group_size_
	and has14_.  Add group_size_.
	(Stub_control::can_add_to_stub_group): Adjust to suit.  Print
	debug info when switching to adding sections before stubs.