re PR libgomp/51298 (libgomp team_barrier locking failures)

	PR libgomp/51298
	* config/linux/bar.h: Use atomic rather than sync builtins.
	* config/linux/bar.c: Likewise.  Add missing acquire
	synchronisation on generation field.
	* task.c (gomp_barrier_handle_tasks): Regain lock so as to not
	double unlock.

From-SVN: r181833