Create a new signal for the Simulator to wait on

Create a new signal for the Simulator to wait on

We wait on "core busy" before simulating an instruction. Trouble is, on a
VL==0 loop, there is no issue, so busy is never toggled. As a solution,
export a new insn_done signal with is pulsed either at end of Execute, or
when going back to Fetch due to skipping a vector instruction.