RFR: 8340838: Clean up MutableCallSite to use explicit release fence instead of AtomicInteger

[Chen Liang]

This implementation code was written in JDK 7, before storeFence was available in JDK 8. We should update this legacy code to make it clear.

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Commit messages:
 - 8340838: Clean up MutableCallSite to use explicit release fence instead of AtomicInteger

Changes: https://git.openjdk.org/jdk/pull/21169/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21169&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340838
  Stats: 5 lines in 1 file changed: 1 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/21169.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21169/head:pull/21169

PR: https://git.openjdk.org/jdk/pull/21169