RFR: 8315856: RISC-V: Use Zacas extension for cmpxchg

[Ludovic Henry]

8315856: RISC-V: Use Zacas extension for cmpxchg

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Commit messages:
 - 8315856: RISC-V: Use Zacas extension for cmpxchg

Changes: https://git.openjdk.org/jdk/pull/16910/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16910&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315856
  Stats: 189 lines in 5 files changed: 159 ins; 3 del; 27 mod
  Patch: https://git.openjdk.org/jdk/pull/16910.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16910/head:pull/16910

PR: https://git.openjdk.org/jdk/pull/16910