RFR: Add missing runtime clone barrier for Shenandoah in new Access API

[Aleksey Shipilev]

On 02/19/2018 11:23 PM, Roman Kennke wrote:
> I forgot to implement the correct runtime barriers in the new Access clone API.
> 
> http://cr.openjdk.java.net/~rkennke/access-clone-barrier/webrev.01/
> 
> Makes specjvm work without throwing all sorts of exceptions.
> 
> Ok?

Yes.

-Aleksey