Layout::linkonce_mapping.
	(Layout::section_name_mapping_count): New variable, replacing
	Layout::linkonce_mapping_count.
	(Layout::linkonce_output_name): Remove.
	(Layout::output_section_name): Rewrite.
	* layout.h (class Layout): Rename Linkonce_mapping to
	Section_name_mapping, linkonce_mapping to section_name_mapping,
	linkonce_mapping_count to section_name_mapping_count.  Don't
	declare linkonce_output_name.