RFR: 8350334: Add final keywords to FFM methods

[Per Minborg]

This PR proposes to add the `final` keyword to some classes and methods in order to simplify compiler devirtualization (e.g. in Graal native image).

Passes `make test TEST=jdk_foreign`

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Commit messages:
 - Make methods and classes final

Changes: https://git.openjdk.org/jdk/pull/23696/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23696&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350334
  Stats: 89 lines in 9 files changed: 5 ins; 0 del; 84 mod
  Patch: https://git.openjdk.org/jdk/pull/23696.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23696/head:pull/23696

PR: https://git.openjdk.org/jdk/pull/23696