Further hacking.

CPU start/stop now only takes effect on cycle boundaries, not mid-cycle.
Previously, if you started CPU 1 from CPU 0, it would run in the same cycle.
But if you started CPU 1 from CPU 2, it wouldn't run until the next cycle.
Now all newly started CPUs run their first instruction in the next cycle,
and halted CPUs only effectively halt at the end of the current cycle.