RFR: 8244551: Shenandoah: Fix racy update of update_watermark

[Roman Kennke]

Disregard this webrev, it contains another change, and Zhengyu also
pointed out that I got volatile HeapWord* wrong: it should be HeapWord*
volatile of course. I'll post another webrev tomorrow.

Thanks,
Roman

> See bug for discussion:
> https://bugs.openjdk.java.net/browse/JDK-8244551
> 
> Fix is to make _update_watermark volatile and access to it ordered
> correctly with load_acquire() and release_store().
> 
> http://cr.openjdk.java.net/~rkennke/JDK-8244551/webrev.00/
> 
> Testing: testcase mentioned in bug on aarch64 several times,
> hotspot_gc_shenandoah
> 
> Ok?
> 
> Roman
>