target-i386: emulate LOCK'ed cmpxchg using cmpxchg helpers

target-i386: emulate LOCK'ed cmpxchg using cmpxchg helpers

The diff here is uglier than necessary. All this does is to turn

FOO

into:

if (s->prefix & PREFIX_LOCK) {
  BAR
} else {
  FOO
}

where FOO is the original implementation of an unlocked cmpxchg.

[rth: Adjust unlocked cmpxchg to use movcond instead of branches.
Adjust helpers to use atomic helpers.]

Signed-off-by: Emilio G. Cota <cota@braap.org>
Message-Id: <1467054136-10430-6-git-send-email-cota@braap.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>