enter exclusive state in destroyJavaVM

This is necessary to ensure that new threads do not start while we're
shutting down (except for the ones that we start to run the shutdown
hooks), and that the shutdown hook threads can be safely started (it
is not safe to start threads from e.g. an idle state, and an assertion
will fail if we do).

src/jnienv.cpp

@@ -79,6 +79,8 @@ destroyJavaVM(Thread* t, uintptr_t*)
     while (t->m->liveCount - t->m->daemonCount > 1) {
       t->m->stateLock->wait(t->systemThread, 0);
     }
+
+    enter(t, Thread::ExclusiveState);
   }
 
   shutDown(t);