sim: Reuse the same limit_event in simulate()

sim: Reuse the same limit_event in simulate()

This patch accomplishes two things:
1. Makes simulate()'s GlobalSimLoopExitEvent a singleton reused
   across calls. This is slightly more efficient than recreating
   it every time.
2. Gives callers to simulate() (especially other simulators) a
   foolproof way of knowing that the simulation period ended
   successfully by hitting the limit event. They can call
   getLimitEvent() and compare it to the return
   value of simulate().

This change was motivated by an ongoing effort to integrate gem5
and SST, with SST as the master sim and gem5 as the slave sim.