Add Illegal conditions for instructions within a HWloop

Created by: davideschiavone

If the core is executing a hardware loop, instructions like:

Uncoditional jumps, conditonal branches, system calls like mret, dret, fences, and wfi are not allowed.

These constraints are alreayd implemented in the RTL. However, they are not in the HWloop constraints Section of the document

https://core-v-docs-verif-strat.readthedocs.io/projects/cv32e40p_um/en/latest/pulp_hw_loop.html#hwloop-specs