cvxif_fu.sv: add register to hold illegal instruction information

Created by: Gchauvon

Hello,

We encountered an issue when illegal instruction were offloaded to CVXIF the same cycle CVXIF sent back result from a previous custom instruction. Illegal instruction was prioritized over the result instruction. The CVA6 would then stall infinitely waiting for this custom instruction result which would never come.

The solution I propose add a register to hold illegal instruction information so we raise the illegal instruction once the CVXIF's result interface is not valid anymore.

Guillaume.