[AArch64] Fix the placement of &_DYNAMIC in the GOT.
	PR gold/15662
	* powerpc.cc (Output_data_brlt_powerpc::reset_brlt_sizes): New
	function.
	(Output_data_brlt_powerpc::finalize_brlt_sizes): New function.
	(Target_powerpc::do_relax): Call the above.
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