Single step and debug cause cleanup

Created by: silabs-oysteink

Removed dependency on other pending debug causes when setting 'pending_single_step.

Updated setting of debug_cause_n, all possible causes are now in the priority (0.13.2, will be updated for 1.0.0). When all casues are set, some code cleanup faning into kill_wb from the DEBUG_TAKEN state was possible.

Updaed RVFI to account for the possibility of have pending_single_step active at the same time as there can be pending async or sync debug reasons.

Signed-off-by: Oystein Knauserud Oystein.Knauserud@silabs.com