RFR: 8244551: Shenandoah: Fix racy update of update_watermark

[Roman Kennke]

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