PR gold/20529 - relaxing loop never ends.

	* powerpc.cc (Stub_table::min_size_threshold_): New member to
	limit size.
	(Stub_table::set_min_size_threshold): New member function.
	(Stub_table::set_address_and_size): Add code to only allow size
	increase.
	(Target_powerpc::do_relax): Add code to record last size.