Fix race condition when shutting down

When we request an auxilary thread to shutdown, maybe another thread is
still waiting to do GC. Therefore we need to guard here so we don't
deadlock when somebody requested a GC but we're also shutting down.

Related to #1952

vm/environment.cpp

@@ -592,7 +592,10 @@ namespace rubinius {
 
     NativeMethod::cleanup_thread(state);
 
-    shared->auxiliary_threads()->shutdown(state);
+    {
+      GCIndependent guard(state);
+      shared->auxiliary_threads()->shutdown(state);
+    }
 
     // Hold everyone.
     while(!state->stop_the_world()) {