[lworld] RFR: 8373375: [lworld] Remove Unsafe.make/finishPrivateBuffer

[Chen Liang]

WIP, replacing make/finishPrivateBuffer with flat array operations, yet to figure out how to ensure UseArrayFlattening is always on...

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Commit messages:
 - Merge branch 'lworld' of https://github.com/openjdk/valhalla into fix/buffer-removal
 - Missed test
 - Rollback breaking change
 - Remove private buffer

Changes: https://git.openjdk.org/valhalla/pull/2018/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=2018&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373375
  Stats: 462 lines in 14 files changed: 6 ins; 416 del; 40 mod
  Patch: https://git.openjdk.org/valhalla/pull/2018.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2018/head:pull/2018

PR: https://git.openjdk.org/valhalla/pull/2018