* powerpc.cc (Target_powerpc::symval_for_branch): Don't assert
	on garbage collected .opd section.

	* powerpc.cc (Target_powerpc::do_gc_add_reference): Test dst_shndx
	is non-zero.
	(Target_powerpc::do_gc_mark_symbols): Likewise for sym->shndx().
	(Target_powerpc::do_function_location): Likewise for loc->shndx.