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

[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.

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Commit messages:
 - Remove redundant import
 - Fix missing fence in `putFlatValueVolatile`
 - Make atomic var handle support depend on user-visible properties
 - Refactor flat value var handle implementation
 - Reintroduce assertion that static fields can't be flat
 - Initial push

Changes: https://git.openjdk.org/valhalla/pull/1402/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1402&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351569
  Stats: 766 lines in 8 files changed: 556 ins; 71 del; 139 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