gas: set up notes obstack earlier

So that the notes obstack can be used for persistent storage in
parse_args.

	* as.c (parse_args): Use notes_alloc and notes_strdup.
	(free_notes): New function.
	(main): Init notes obstack, and arrange to be freed on exit.
	* read.c (read_begin): Don't init notes obstack.
	(read_end): Free cond_obstack.
	* subsegs.c (subsegs_end): Don't free cond_obstack or notes.