Cleanup NEWUNIT allocation.

2011-11-01  Janne Blomqvist  <jb@gcc.gnu.org>

	* io/io.h (next_available_newunit): Remove prototype.
	* io/unit.h (next_available_newunit): Make variable static,
	initialize it.
	(init_units): Don't initialize next_available_newunit.
	(get_unique_unit_number): Use atomic builtin if available.

From-SVN: r180734