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)