Print unattached sections in the link map.

gold/
	PR gold/16980
	* layout.cc (Layout::print_to_mapfile): Print unattached sections in
	map.
Add check to keep from segfaulting on a corrupt .debug_pubnames section.

gold/
	* dwarf_reader.cc (Dwarf_pubnames_table::read_header): Check that
	unit_length is within section bounds.