cpu: Query CPU for inst executed from Python

This patch adds the ability for the simulator to query the number of
instructions a CPU has executed so far per hw-thread. This can be used
to enable more flexible periodic events such as taking checkpoints
starting 1s into simulation and X instructions thereafter.

src/cpu/BaseCPU.py

@@ -101,6 +101,7 @@ class BaseCPU(MemObject):
     Counter totalInsts();
     void scheduleInstStop(ThreadID tid, Counter insts, const char *cause);
     void scheduleLoadStop(ThreadID tid, Counter loads, const char *cause);
+    uint64_t getCurrentInstCount(ThreadID tid);
 ''')
 
     @classmethod