[foreign-memaccess+abi] RFR: Tighten up sealed/final classes

[Per Minborg]

This PR tightens up the way implementing classes are sealed and subclassable. Only internal classes are involved in this PR.

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Commit messages:
 - Tighten up sealed/final classes

Changes: https://git.openjdk.org/panama-foreign/pull/709/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=709&range=00
  Stats: 20 lines in 5 files changed: 4 ins; 2 del; 14 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/709.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/709/head:pull/709

PR: https://git.openjdk.org/panama-foreign/pull/709