Mips: Omit empty objects for merging processor-specific data.

Mips: Omit empty objects for merging processor-specific data.

gold/
	* mips.cc (Mips_relobj::merge_processor_specific_data_): New data
	member.
	(Mips_relobj::merge_processor_specific_data): New method.
	(Mips_relobj::do_read_symbols): Set merge_processor_specific_data_
	to false, only if the input file is a binary or if object has no
	contents except the section name string table and an empty symbol
	table with the undefined symbol.
	(Target_mips::do_finalize_sections): Refactor.  Skip empty object files
	for merging processor-specific data.