BR 3392279: Fix duplicated REX prefixes

The fix for BR 3392278:

aa29b1d93f5a 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>
doc: Document --v and duplicate REX prefix fix

Document changes for 2.11.05.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
NASM 2.11.05