Retry powerpc gold stub grouping when groups prove too large

Retry powerpc gold stub grouping when groups prove too large

An unusually large number of stubs can result in the default section
group size being too large;  sections plus stubs exceed the range of a
branch.  Restarting the relaxation pass with a smaller group size can
sometimes help.

	* powerpc.cc (struct Stub_table_owner): New.
	(Powerpc_relobj): Rename stub_table_ to stub_table_index_, an
	unsigned int vector.  Update all references.
	(powerpc_relobj::set_stub_table): Take an unsigned int param
	rather than a Stub_table.  Update callers.
	(Powerpc_relobj::clear_stub_table): New function.
	(Target_powerpc): Add relax_failed_, relax_fail_count_ and
	stub_group_size_ vars.
	(Target_powerpc::new_stub_table): Delete.
	(max_branch_delta): New function, extracted from..
	(Target_powerpc::Relocate::relocate): ..here..
	(Target_powerpc::Branch_info::make_stub): ..and here.  Return
	status on whether stub created successfully.
	(Stub_control::Stub_control): Add "no_size_errors" param.  Move
	default sizing to..
	(Target_powerpc::do_relax): ..here.  Init stub_group_size_ and
	reduce on relax failure.
	(Target_powerpc::group_sections): Add "no_size_errors" param.
	Use stub_group_size_.  Set up group info in a temp vector,
	before building Stub_table vector.  Account for input sections
	possibly already converted to relaxed sections.
	(Stub_table::init): Delete.  Merge into..
	(Stub_table::Stub_table): ..here.
	(Stub_table::can_reach_stub): New function.
	(Stub_table::add_plt_call_entry): Add "from" parameter and
	return true iff stub could be reached.
	(Stub_table::add_long_branch_entry): Similarly.  Add "r_type"
	param too.
	(Stub_table::clear_stubs): Add "all" param.