The fix for BR 3392278:
aa29b1d93f assemble.c: Don't drop rex prefix from instruction itself
... would cause multiple REX prefixes to be emitted for some
instructions. Create a new flag to indicate that REX has already been
emitted, which can be cleared for each instance of an instruction.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
