PR21503, Gold doesn't create linker stub symbols on ppc64

PR21503, Gold doesn't create linker stub symbols on ppc64

	PR 21503
	* options.h: Add --emit-stub-syms option.
	* powerpc.cc (object_id): New.
	(Powerpc_relobj): Add uniq_ and accessor.  Sort variables for
	better packing.
	(Powerpc_dynobj): Sort variables for better packing.
	(Target_powerpc::define_local): New function.
	(Target_powerpc::group_sections): Pass stub table size to
	Stub_table constructor.
	(Target_powerpc::do_relax): Define stub and glink symbols.
	(Stub_table): Add uniq_ variable, and id param to constructor.
	(Stub_table::Plt_stub_ent): Add indx_ variable.
	(Stub_table::Branch_stub_entries): Move typedef earlier.
	(Stub_table::branch_stub_size): Replace "to" parameter with a
	Branch_stub_entries iterator.
	(Stub_table::add_long_branch_entry): Adjust to suit.
	(Stub_table::add_plt_call_entry): Set indx_.
	(Stub_table::define_stub_syms): New function.