Revert thread_local stuff

Unfortunatly accessing thread local variable
is much slower than object data (see previous
patch log msg), so we have to revert to old code
to avoid speed regression.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/benchmark.cpp

@@ -107,7 +107,7 @@ void benchmark(istringstream& is) {
 
   for (size_t i = 0; i < fens.size(); i++)
   {
-      Position pos(fens[i], false);
+      Position pos(fens[i], false, NULL);
 
       cerr << "\nPosition: " << i + 1 << '/' << fens.size() << endl;