RFR: 8295948: Support for Zicbop/prefetch instructions on RISC-V [v4]

[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 two additional commits since the last revision:

 - fixup! remove dead code
 - remove dead code

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/10884/files
  - new: https://git.openjdk.org/jdk/pull/10884/files/0c8691b9..48ab8f31

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10884&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10884&range=02-03

  Stats: 57 lines in 4 files changed: 0 ins; 57 del; 0 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