Move the wait on "core stop" out of fetch, and into issue

Move the wait on "core stop" out of fetch, and into issue

During a Simple-V loop, the fetch FSM will sit idle, unable to pause the
execution. A good alternate place to wait on "core stop" release is at
instruction end, before either fetching a new instruction, or going back
to the SV loop.

On system initialization, we need to pause as well, since there was no
instruction which ended previously.