target/i386: implement CMPccXADD

target/i386: implement CMPccXADD

The main difficulty here is that a page fault when writing to the destination
must not overwrite the flags.  Therefore, the flags computation must be
inlined instead of using gen_jcc1*.

For simplicity, I am using an unconditional cmpxchg operation, that becomes
a NOP if the comparison fails.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>