[lworld] Integrated: Adjustment and new API for VarHandles

[Frederic Parain]

A few changes related to the work to fix the VarHandles code.
The keyword volatile is now a signal to prevent field flattening.
A new API in ValueClass allows Java code to query the atomicity characteristics of an array.

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Commit messages:
 - Adjustment and new API for VarHandles

Changes: https://git.openjdk.org/valhalla/pull/1401/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1401&range=00
  Stats: 162 lines in 8 files changed: 156 ins; 3 del; 3 mod
  Patch: https://git.openjdk.org/valhalla/pull/1401.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1401/head:pull/1401

PR: https://git.openjdk.org/valhalla/pull/1401