Add some missing UseShenandoahGC checks to 8u

[Roland Westrelin]

> Why not 9 and 10?

Sure. But it seemed to me it was easier and more urgent to start with
8u.

So, for 9, do we agree that big and invasive changes like strip mining
that require some refactoring and so would be too risky to ship anyway,
should be removed?

And, for 10, do we agree that there's no need to make non shenandoah
specific changes that are expected to land upstream conditional under
UseShenandoahGC?

Roland.