RFR: 8296463: Memory leak in JVM_StartThread with the integration of Virtual threads

[David Holmes]

Please review this simple fix that removes a memory leak. The _extentLocalCache OopHandle was not being released when a JavaThread was destroyed.

Testing:
 - manual leak checking with NMT
 -  tier1 (sanity)
Thanks.

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Commit messages:
 - 8296463: Memory leak in JVM_StartThread with the integration of Virtual threads

Changes: https://git.openjdk.org/jdk/pull/11198/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11198&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296463
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11198.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11198/head:pull/11198

PR: https://git.openjdk.org/jdk/pull/11198