[rtl] Ibex services interrupts when single stepping

[GregAC]

Ibex hard-codes dcsr.stepie to 0, meaning when single-stepping is enabled all interrupts should be masked. From trials with debugging on https://github.com/GregAC/ibex_super_system this is not the observed behaviour, a single step can enter an instruction handler.

In addition to fixing this we should look at our existing DV to determine if this bug could have been caught by it (single step tests are failing in the nightly regression so potentially have already found the bug, we just haven't had the time to look at the failures yet)

Increased estimate as I've spent time on testing this and not yet finished. Fix is available as a PR.
old estimate 4

estimate 6
remaining 2022-09-23 4
remaining 2022-09-30 2

[GregAC]

@hcallahan-lowrisc have you come across any behaviour around single stepping and interrupts in your debug DV work?