sim: Fix hang while terminating threads with fork

It is possible that gem5 is forked multiple times before the threads are
re-created, so don't wait for the barrier if the threads were already
terminated.

Change-Id: Ia04db2f3b1341c4d432178a37812fd882e148ec2
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50710
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/sim/simulate.cc

@@ -118,6 +118,8 @@ class SimulatorThreads
     terminateThreads()
     {
         assert(!terminate);
+        if (threads.empty())
+            return;
 
         /* This function should only be called when the simulator is
          * handling a global exit event (typically from Python). This