[GOLD] PowerPC linkage table error

[GOLD] PowerPC linkage table error

This fixes a segfault when attempring to output a "linkage table
error".  "object" is only non-NULL in the local symbol case.

	* powerpc.cc (Stub_table::plt_error): New function.
	(Stub_table::do_write): Use it.
	(Output_data_glink::do_write): Don't segfault emitting linkage
	table error.