[lworld] RFR: 8351483: [lworld] Field offset verification fails

[Frederic Parain]

Fixing generation of flat layouts, to generate them only if some flattening is enabled.
Fixing the wrapper classes offsets verification code.

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Commit messages:
 - Fix layouts generation and offsets verification

Changes: https://git.openjdk.org/valhalla/pull/1393/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1393&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351483
  Stats: 100 lines in 3 files changed: 97 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/valhalla/pull/1393.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1393/head:pull/1393

PR: https://git.openjdk.org/valhalla/pull/1393