Revisit JIT options

We disabled the JIT options as a test fir #9387 (moved). We should re-enable them either when the security slider is complete, or perhaps earlier. Georg and I spoke with a Mozilla engineer who recommended a write protection patch for the JIT, and recommended that the baseline jit be enabled. We might be able to do that before the security slider.

This ticket will be the parent ticket for all of the jit issues, to keep that conversation out of the security slider ticket.