Test CMPXCHG-FAIL-UNLOCK

X86_64 CMPXCHG-FAIL-UNLOCK
{
int x;
int y;
0:rax=2;
0:rbx=1;
}

 P0               | P1             ;
 movl $1,(x)      | movl $1,(y)    ;
 cmpxchg (z),%ebx | mfence         ;
 movl (y),%ecx    | movl (x),%ecx  ;
exists (z=0 /\ 0:rcx=0 /\ 1:rcx=0)