Implement proper mret when mcause.minhv==1

Created by: silabs-oysteink

If mcause.minhv is set when we execute an mret, we shall not restore the pc to the contents of mepc, but instead do a pointer fetch M[mepc] as for the CLIC pointer fetch as a data access with lsu privilege.

To achieve this, the pipeline must be stalled until EX and WB are empty before starting the pointer fetch. This is because any instructions in EX or WB may still cause exceptions, and the pointer fetch would use the state POINTER_FETCH in the controller - which does not handle exceptions.