target-i386: fix cmpxchg instruction emulation

target-i386: fix cmpxchg instruction emulation

When the i386 cmpxchg instruction is executed with a memory operand
and the comparison result is "unequal", do the memory write before
changing the accumulator instead of the other way around, because
otherwise the new accumulator value will incorrectly be used in the
comparison when the instruction is restarted after a page fault.

This bug was originally reported on 2010-04-25 as
https://bugs.launchpad.net/qemu/+bug/569760

Signed-off-by: Andreas Gustafsson <gson@gson.org>