RFR: 8295948: Support for Zicbop/prefetch instructions on RISC-V

[Ludovic Henry]

The OpenJDK supports generating prefetch instructions on most platforms. RISC-V supports through the Zicbop extension the use of prefetch instructions. We want to make sure we use these instructions whenever they are available.

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Commit messages:
 - 8295948: Support for Zicbop/prefetch instructions on RISC-V

Changes: https://git.openjdk.org/jdk/pull/10884/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10884&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295948
  Stats: 134 lines in 7 files changed: 130 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/10884.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10884/head:pull/10884

PR: https://git.openjdk.org/jdk/pull/10884