fix deadlock MySystem::visit in posix.cpp

We must call notifyAll on visitLock after setting threadVisitor to
null in case another thread is waiting to do a visit of its own.
Otherwise, the latter thread will wait forever, eventually deadlocking
the whole VM at the next GC since it's in an active state.

src/posix.cpp

@@ -655,6 +655,8 @@ class MySystem: public System {
 
       threadVisitor = 0;
 
+      system->visitLock->notifyAll(t);
+
       return 0;
     } else {
       return -1;