sim: Make MaxTick in Python match the one in C++

This patch aligns the MaxTick in Python with the one in C++. Thus,
both reflect the maximum value that an unsigned 64-bit integer can
have.

src/python/m5/simulate.py

@@ -57,8 +57,8 @@ from m5.internal.stats import updateEvents as updateStatEvents
 from util import fatal
 from util import attrdict
 
-# define a MaxTick parameter
-MaxTick = 2**63 - 1
+# define a MaxTick parameter, unsigned 64 bit
+MaxTick = 2**64 - 1
 
 _memory_modes = {
     "atomic" : objects.params.atomic,