RFC: Pick up jdk-13+18 to sh/jdk

[Roman Kennke]

Ok let's do it.!

Roman


> We have not synced up sh/jdk for a while now. Let's pick up to the latest upstream tag, jdk-13+18.
> Merge is trivial. This should fix jcstress tests in sh/jdk (missing cases in get_barrier_strength).
> 
> Changesets:
>    http://cr.openjdk.java.net/~shade/shenandoah/merges/jdk-13%2b18/changesets.txt
> 
> Testing: hotspot_gc_shenandoah {fastdebug, release}
>