stats: Provide a mechanism to get a callback when stats are dumped.

This mechanism is useful for dumping output that is correlated with stats
dumping, but isn't tracked by the gem5 statistics.

src/python/m5/stats/__init__.py

@@ -103,6 +103,8 @@ def dump():
         return
     lastDump = curTick
 
+    internal.stats.processDumpQueue()
+
     prepare()
 
     for output in outputList:

src/python/swig/stats.i

@@ -71,6 +71,13 @@ processResetQueue()
     resetQueue.process();
 }
 
+inline void
+processDumpQueue()
+{
+    extern CallbackQueue dumpQueue;
+    dumpQueue.process();
+}
+
 inline char *
 PCC(const char *string)
 {
@@ -146,6 +153,7 @@ void schedStatEvent(bool dump, bool reset,
                     Tick when = curTick(), Tick repeat = 0);
 
 void processResetQueue();
+void processDumpQueue();
 void enable();
 bool enabled();