Cherry pick native barrier related changes to shenandoah/jdk

[Roman Kennke]

>> I just pushed my last-week's merge which includes the first two of your
>> cherry-picks and you may want to wait for my 8227637 cherry-pick too, so
>> that we can finally have an unbroken sh/jdk.
> 
> Okay, the list down to 2 items:
> 
> Cherry-pick: 8226695: Shenandoah: Wire C1 and C2 IN_NATIVE barrier
> Cherry-pick: 8225483: Shenandoah: Enhance native access barrier
> 
> Webrev:
> http://cr.openjdk.java.net/~zgu/shenandoah/native_barriers/webrev.01/
> 
> Test:
>   hotspot_gc_shenandoah (fastdebug and release)

Ok. Thanks!

Roman