RFR: 8347366: RISC-V: Add extension asserts for CMO instructions

[Robbin Ehn]

Hi please consider this minor improvement.

Sanity tested.

Thanks, Robbin

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Commit messages:
 - Added assert, comments and used templates

Changes: https://git.openjdk.org/jdk/pull/23015/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23015&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347366
  Stats: 51 lines in 1 file changed: 21 ins; 0 del; 30 mod
  Patch: https://git.openjdk.org/jdk/pull/23015.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23015/head:pull/23015

PR: https://git.openjdk.org/jdk/pull/23015