Standard Switch: Drain the system before switching CPUs

When switching from an atomic CPU to any of the timing CPUs, a drain is
unnecessary since no events are scheduled in atomic mode. However, when
trying to switch CPUs starting with a timing CPU, there may be events
scheduled. This change ensures that all events are drained from the system
by calling m5.drain before switching CPUs.

src/python/m5/simulate.py

@@ -203,7 +203,6 @@ def changeToTiming(system):
               (type(system), objects.Root, objects.System)
 
     if system.getMemoryMode() != objects.params.timing:
-        doDrain(system)
         print "Changing memory mode to timing"
         for obj in system.descendants():
             obj.changeTiming(objects.params.timing)