RFR: 7904041: jcstress: Amend seqcst tests with reference types

[Aleksey Shipilev]

[JDK-8351997](https://bugs.openjdk.org/browse/JDK-8351997) shows there is a gap in testing: we only verify seqcst scenarios over primitive volatiles. We need to extend this to reference types.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jcstress/pull/172/files
  Webrev: https://webrevs.openjdk.org/?repo=jcstress&pr=172&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904041
  Stats: 39 lines in 3 files changed: 31 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jcstress/pull/172.diff
  Fetch: git fetch https://git.openjdk.org/jcstress.git pull/172/head:pull/172

PR: https://git.openjdk.org/jcstress/pull/172