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
Merge branch 'nasm-2.11.xx'

Resolved Conflicts:
	assemble.c

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
doc: Make it clear that --v is only in 2.11.05 and newer

... otherwise people might think it is preferred and use it
unnecessarily.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Merge branch 'nasm-2.11.xx'