riscv: gate Zcb instructions on their required extensions

[carlosqwqqwq]

Summary

The Zcb dispatcher checks only Zcb and then translates forms that additionally require Zbb, M or Zmmul. Add the per-instruction prerequisite checks before lowering those forms, preserving legal controls and illegal traps.

Validation

• The four representative raw encodings trap in the missing-prerequisite profiles after the fix; legal controls remain valid.
• The patch is limited to the Zcb dispatcher.

Fixes renode/renode#960