Add OpSize16 bit, for instructions which need 0x66 prefix in 16-bit mode

The 0x66 prefix toggles between 16-bit and 32-bit addressing mode.
So in 32-bit mode it is used to switch to 16-bit addressing mode for the
following instruction, while in 16-bit mode it's the other way round — it's
used to switch to 32-bit mode instead.

Thus, emit the 0x66 prefix byte for OpSize only in 32-bit (and 64-bit) mode,
and introduce a new OpSize16 bit which is used in 16-bit mode instead.

This is just the basic infrastructure for that change; a subsequent patch
will add the new OpSize16 bit to the 32-bit instructions that need it.

Patch from David Woodhouse.