nasm.c: min 2 optimization passes, don't re-run pass 1
We may not even have the most basic stabilization done unless we run
at least two optimization passes, e.g. in the case of subsections.
However, we cannot run more than one stabilization pass (pass0 == 1);
for one thing we'll call ofmt->symdef() multiple times on the same
symbol, which is not allowed. If we haven't achieved stability by the
time we decide to run a stabilization pass, plod on and hope for the
best.
Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>