RFR (S): Add the option to disable Shenandoah SATB barriers

[Andrew Haley]

On 20/03/17 09:02, Aleksey Shipilev wrote:
> Introducing ShenandoahSATBBarriers option:
>   http://cr.openjdk.java.net/~shade/shenandoah/satb-barriers-disable/webrev.02/

Some of these changes are very intrusive in shared code.  I suppose
that when we get a proper abstract GC interface they will all be
abstracted away, so perhaps it doesn't matter.

Andrew.