Bug in data vs. addr dependency calculation.

* Notice that the solver may let pass event structures with
  unresolved variables, consider classical out-of-thin-air for instance:
PPC LB
{
0:r2=x; 0:r4=y;
1:r2=y; 1:r4=x;
}
 P0           | P1           ;
 lwz r1,0(r2) | lwz r1,0(r2) ;
 stw r1,0(r4) | stw r1,0(r4) ;
locations [0:r1; 1:r1;]

Then, herd gives variable S7 in output....
Interpret the Rmw "pseudo candidate relaxation" as C atomic_exchange.

Notice: also changed the meaning of Hat -> external
Merge commit '411e13dcdfcf3b6889adf99e71d3dc69c0b09bc5' into johnc3

Conflicts:
	litmus/CTarget.mli