iflag: In iflag_cmp() scan from the most significant word down

iflag: In iflag_cmp() scan from the most significant word down

In order for iflag_cmp() to return an ordering that makes sense, we
need to scan from the most significant word downward.  That way the
bits with the higher index consistently are the more significant.

This fixes the disassembler vendor selection algorithm.  While we are
doing that, make that dependency more explicit in the comments.

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