RFR: 8295948: Support for Zicbop/prefetch instructions on RISC-V [v8]

[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.
> 
> It passes `hotspot:tier1` test suite

Ludovic Henry has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10884/files
  - new: https://git.openjdk.org/jdk/pull/10884/files/42a61aa8..0e92909e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10884&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10884&range=06-07

  Stats: 8 lines in 1 file changed: 2 ins; 0 del; 6 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