Minor tweaks to make a switchover at tick N match

Minor tweaks to make a switchover at tick N match
restarting from a checkpoint at tick N.

cpu/simple_cpu/simple_cpu.cc:
    On a CPU switchover, schedule the new CPU's first tick event
    for curTick+1 instead of curTick.

--HG--
extra : convert_revision : f0757d6f028214d36c1cff992db688fd6e8a6fdc