RFR: 8321470: ThreadLocal.nextHashCode can be static final

[Brett Okken]

The static AtomicInteger used for the nextHashCode should be final.

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Commit messages:
 - Merge remote-tracking branch 'upstream/master' into threadlocal_final
 - make ThreadLocal.nextHashCode final

Changes: https://git.openjdk.org/jdk/pull/16987/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16987&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321470
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16987.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16987/head:pull/16987

PR: https://git.openjdk.org/jdk/pull/16987