RFR: 8376664: Find a better place for the Atomic<HeapWord*> vmstructs toplevel declaration

[Thomas Schatzl]

Hi all,

  please review this change that moves the declaration of `Atomic<HeapWord*>` to gc specific vmstructs, replacing the old similarly used `HeapWord* volatile`. This is a much better place.

Testing: gha

Thanks,
  Thomas

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Commit messages:
 - 8376664

Changes: https://git.openjdk.org/jdk/pull/29650/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29650&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8376664
  Stats: 2 lines in 2 files changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/29650.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29650/head:pull/29650

PR: https://git.openjdk.org/jdk/pull/29650