Replace curTick global variable with accessor functions.

This step makes it easy to replace the accessor functions
(which still access a global variable) with ones that access
per-thread curTick values.

src/python/m5/simulate.py

@@ -127,7 +127,7 @@ def simulate(*args, **kwargs):
 
 # Export curTick to user script.
 def curTick():
-    return internal.core.cvar.curTick
+    return internal.core.curTick()
 
 # Python exit handlers happen in reverse order.  We want to dump stats last.
 atexit.register(internal.stats.dump)

src/python/swig/core.i

@@ -73,8 +73,7 @@ const bool flag_TRACING_ON;
 
 void setClockFrequency(Tick ticksPerSecond);
 
-%immutable curTick;
-Tick curTick;
+Tick curTick();
 
 class Checkpoint;
 

src/python/swig/stats.i

@@ -37,6 +37,7 @@
 #include "base/statistics.hh"
 #include "base/stats/mysql.hh"
 #include "base/stats/text.hh"
+#include "sim/core.hh"
 #include "sim/stat_control.hh"
 %}
 
@@ -51,7 +52,7 @@ void initMySQL(std::string host, std::string database, std::string user,
     std::string sample);
 
 void schedStatEvent(bool dump, bool reset,
-                    Tick when = curTick, Tick repeat = 0);
+                    Tick when = curTick(), Tick repeat = 0);
 
 void enable();
 void prepare();