x86: pre-process opcodes table before parsing

x86: pre-process opcodes table before parsing

Instead of expanding macro-like constructs in i386-gen, have the C pre-
processor do this for us. Besides being a prerequisite for the next
template folding steps, this also paves the way for removing various
hidden dependencies between #define-s in i386-opc.h and plain literal
numbers used in i386-opc.tbl.

The #undef of None is solely to leave the generated i386-tbl.h entirely
unchanged.