add check for legal funct3 for IW instructions

Created by: AlecVercruysse

Fixes #152 (closed).

This was tested by using the ImperasDV lockstep simulator:

#include "WALLY-init-lib.h"

main:
    # Test illegal instructions are detected
    .word 0x0000701B  // Illegal IW instruction

    j done

The test failed for the current upstream and worked for this branch.