[11] 2020-05-08, Backports to sh/jdk11

[Roman Kennke]

Looks good, thanks!

Roman

> http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk11-20200508/webrev.01/
> 
> This backports two critical bugfixes to sh/jdk11:
>  [backport] 8244551: Shenandoah: Fix racy update of update_watermark
>  [backport] 8244200: Shenandoah: build breakages after JDK-8241743
> 
> I had to change Atomic -> OrderAccess to make it compile.
> 
> Testing: hotspot_gc_shenandoah {fastdebug, release}
>