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

[Aleksey Shipilev]

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}

-- 
Thanks,
-Aleksey