target-i386: emulate LOCK'ed NEG using cmpxchg helper

[rth: Move redundant qemu_load out of cmpxchg loop.]

Signed-off-by: Emilio G. Cota <cota@braap.org>
Message-Id: <1467054136-10430-16-git-send-email-cota@braap.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>
