dekker-fences: add Dekker's critical section algorithm

dekker-fences: add Dekker's critical section algorithm

Now that we support fences, add a test benchmark that uses fences. This
one just uses seq-cst fences.

Downloaded from:

   http://www.justsoftwaresolutions.co.uk/threading/

Modified to avoid static construction. i.e., change:
  std::atomic<int> turn(0);
to
  std::atomic<int> turn;
  ...
  turn = 0;
And added user_main() method to launch the two threads.