x86: drop Rm and the 'L' macro

Rm (and hence OP_R()) can be dropped by making 'Z' force modrm.mod to 3
(for OP_E()) instead of ignoring it. While at it move 'Z' handling to
its designated place (after 'Y'; 'W' handling will be moved by a later
change).

Moves to/from TRn are illegal in 64-bit mode and thus get converted to
honor this at the same time (also getting them in line with moves
to/from CRn/DRn ModRM.mod handling wise). This then also frees up the L
macro.