* options.h (General_options): Add plt_align, plt_static_chain,

	* options.h (General_options): Add plt_align, plt_static_chain,
	plt_thread_safe.  Update stub_group_size help text.
	* powerpc.cc (Target_powerpc::plt_thread_safe): New access function
	for new plt_thread_safe_ var.
	(use_plt_offset): Correct comments.
	(Target_powerpc::do_relax): Look for thread creation symbols to
	determine default plt_thread_safe value.  Clear plt call stubs
	as well as branch stubs each iteration.
	(add_2_2_11, add_12_12_11, bnectr_p4, cmpldi_2_0, xor_11_11_11): New
	insn constants.
	(l, hi, ha, write_insn): Move earlier.
	(Stub_table): Delete prev_size, add last_plt_size and last_branch_size.
	(Stub_table::clear_stubs): Rename from clear_long_branch_stubs, clear
	plt stubs too.
	(Stub_table::update_size): Adjust.
	(Stub_table::prev_size, set_prev_size): Delete.
	(Stub_table::stub_align): Let --plt-align affect result.
	(Stub_table::plt_call_size): Calculate sizes for various stubs.
	(Stub_table::branch_stub_size): Use last_plt_size in address calc.
	(Stub_table::add_plt_call_stub): Pass iterator to plt_call_size.
	(Stub_table::do_write): Support more stub variants.