Correct our implementation of Thread.VolatileRead ()/VolatileWrite ().

We have various issues:

* All of the functions assumed that doing volatile loads/stores was
  enough, when load-acquire, store-release semantics were actually
  needed. This may have worked before purely by chance. We now use
  proper memory barriers so that we don't have to hope for the
  compiler and CPU to do the right thing.
* Removes the hack for 64-bit quantities on 32-bit systems. This is
  no longer needed now that we use explicit memory barriers in
  these functions. Also, these functions are not supposed to do
  atomic reads/writes in the first place - they're purely about the
  memory barrier semantics.
* The VolatileWrite (object&, object) overload was not using the
  volatile qualifier at all, thus not getting volatile semantics
  as per: http://gcc.gnu.org/onlinedocs/gcc/Volatiles.html