[GOLD] ppc64 out-of-line register save/restore functions
    
    Gold version of a4b6fadd.  Don't emit long branch or plt branch stubs
    to save/restore functions.  Copy them instead.
    
    	PR 18878
    	* powerpc.cc (Target_powerpc): Add savres_section_ and accessor.
    	(Target_powerpc::Branch_info::make_stub): Determine whether long
    	branch stub is for save/restore function.
    	(Branch_stub_ent): Add save_res_, and extra parm to constructor.
    	(Stub_table): Add need_save_res_.
    	(Stub_table:clear_stubs): Clear need_save_res_.
    	(Stub_table:set_address_and_size): Add save/restore section size.
    	(Stub_table::add_long_branch_entry): Add save_res param.  Set
    	need_save_res_, but don't add space for save/restore stubs.
    	(Stub_table::find_long_branch_entry): Return offset to local copy
    	of save/restore func.
    	(Stub_table::do_write): Don't output save/restore stubs.  Instead
    	copy the save/restore functions.
    	(Output_data_save_res:contents): New accessor.
    	(Target_powerpc::define_save_restore_funcs): Set savres_section_.