RFR: 8343555: RISC-V: make UseZvfh diagnostic option

[Hamlin Li]

Hi,
Can you help to review this simple patch?
Previously this UseZvfh was changed from experimental to product because we have real hardware to verify the feature, but as pointed out by @RealFYang , this one should ber a diagnostic option, as we don't want to expose too many options to users.
Thanks

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Commit messages:
 - Initial commit
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Revert "initial commit"
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/21885/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21885&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343555
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21885.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21885/head:pull/21885

PR: https://git.openjdk.org/jdk/pull/21885