[lworld] RFR: 8351569: [lworld] Revisit atomic access modes in flat var handles [v3]

[Maurizio Cimadamore]

> This PR is an attempt to put var handle support for flat values on a more solid footing. Some more notes in a comment below.

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Fix test header

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/1402/files
  - new: https://git.openjdk.org/valhalla/pull/1402/files/8ca6bef1..1cd81764

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1402&range=02
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1402&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/valhalla/pull/1402.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1402/head:pull/1402

PR: https://git.openjdk.org/valhalla/pull/1402