Fix newly-introduced Thread#kill deadlock
The following commit introduced another deadlock. Threads MUST be Rubinius.synchronize-d, but they weren't in Thread#kill. ba7667b Correctly implement Thread#kill Previously, Thread#kill internally used Thread#raise, which had gotten completely deadlock-free. And there is no problem. But, the commit made Thread#kill use a primitive for its function. That must be inside Rubinius.synchronize as was before.