Add support for instructions which always use low 8-bit registers

Add support for instructions which always use low 8-bit registers

Add a byte code to explicitly support instructions which only uses the
low 8-bit registers (as if a REX prefix always was present.)  This is
usable for instructions which are officially documented as using "the
low byte of a 32-bit register" and so on.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>