Implement late forwarding for MADD

[mbitsnbites]

With late forwarding of the addend, the MADD instruction would work as an MAC with zero latency for consecutive multiply+add operations such as:

    madd  r1, r2, r3
    madd  r1, r4, r5
    madd  r1, r6, r7  ; r1 = r1 + r2 * r3 + r4 * r5 + r6 * r7

We probably only need to worry about forwarding of outputs from the MADD unit.