The Fiber thread field does not need to be volatile

* It is always read from the Fiber's thread,
  except in Thread#wakeup which is inherently racy.
* Around 2 billion iterations/s now for
  benchmark { begin; raise "foo"; rescue; end }.

Author: eregon

src/main/java/org/truffleruby/core/fiber/FiberLayout.java

@@ -34,7 +34,7 @@ DynamicObject createFiber(DynamicObjectFactory factory,
                               DynamicObject rubyThread,
                               @Volatile @Nullable DynamicObject lastResumedByFiber,
                               @Volatile boolean alive,
-                              @Volatile @Nullable Thread thread,
+                              @Nullable Thread thread,
                               @Volatile boolean transferred);
 
     boolean isFiber(DynamicObject object);

src/main/java/org/truffleruby/core/thread/ThreadNodes.java

@@ -430,7 +430,7 @@ public DynamicObject wakeup(DynamicObject rubyThread,
                                     @Cached("new()") YieldNode yieldNode) {
             final DynamicObject currentFiber = Layouts.THREAD.getFiberManager(rubyThread).getCurrentFiberRacy();
             final Thread thread = Layouts.FIBER.getThread(currentFiber);
-            if (thread == null) {
+            if (!Layouts.FIBER.getAlive(currentFiber) || thread == null) {
                 throw new RaiseException(getContext(), coreExceptions().threadErrorKilledThread(this));
             }