Add a symbols-only mode to nm.

Add a symbols-only mode to nm.

	PR 27487
	* nm.c (FORMAT_JUST_SYMBOLS): Define.
	(struct optput_fns): Add entry for FORMAT_JUST_SYMBOLS.
	(long_options): Add just-symbols.
	(set_output_format): Add support for just-symbols.
	(get_print_format): Likewise.
	(do_not_print_object_filename): New function.
	(do_not_print_archive_filename): New function.
	(do_not_print_archive_member): New function.
	(do_not_print_symbol_filename): New function.
	(just_print_symbol_name): New function.
	(main): Handle --just-symbols.
	* NEWS: Mention the new feature.
	* doc/binutils.texi: Document the new feature.