Relax G1EvacStats atomic operations

G1EvacStats uses atomic operations like:
  Atomic::add(&_direct_allocated, value);

This causes the default barriers to be used:

  // Returns updated value.
  template<typename D, typename I>
  inline static D add(D volatile* dest, I add_value,
                      atomic_memory_order order = memory_order_conservative);

Since these are stats, we can use memory_order_relaxed and get a tiny improvement for gathering stats. (I doubt it would show on benchmarks, this is a paper-cut issue.)