[X86] Correct the implementation of ud1(a.k.a. ud2b) instruction.

[X86] Correct the implementation of ud1(a.k.a. ud2b) instruction.

We were missing the modrm byte this instruction has according
to current Intel SDM. Experiments with gcc indicate that different
modrm values are chosen based on 2 operands so I've added those
as well.

I think our previous implementation was based on an older behavior of
binutils that has since been changed.