x86: correct UDn

x86: correct UDn

Make the assembler recognize UD0, supporting only the newer form
expecting a ModR/M byte.

Make assembler and disassembler properly emit / expect a ModR/M byte for
UD1.

For the testsuite, as arch-4 already tests all UDn, avoid producing a
huge delta for other tests using UD2B by making them use UD2 instead.