gcc.c: Document new %@{...} sequence.

gcc.c: Document new %@{...} sequence.

	* gcc.c: Document new %@{...} sequence.
	(LINK_COMMAND_SPEC): Use it for the -L switches.
	(cpp_unique_options): Use it for the -I switches.
	(at_file_argbuf): New global variable.
	(in_at_file): Likewise.
	(alloc_args): Create at_file_argbuf.
	(clear_args): Truncate at_file_argbuf.
	(store_arg): If in_at_file, push the argument onto at_file_argbuf.
	(open_at_file): New function.
	(close_at_file): Likewise.
	(create_at_file): Delete.
	(do_spec_1) <'i'>: Use open_at_file/close_at_file.
	<'o'>: Likewise.
	<'@'>: New case.
	(validate_switches_from_spec): Deal with %@{...} sequence.
	(validate_switches): Likewise.
	(driver::finalize): Call clear_args.

From-SVN: r261474