RFR: 8343555: RISC-V: make UseZvfh diagnostic option [v2]

[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

Hamlin Li has updated the pull request incrementally with one additional commit since the last revision:

  turn more verified extensions as DIAGNOSTIC

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/21885/files
  - new: https://git.openjdk.org/jdk/pull/21885/files/4b41bb91..e5bd3eef

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21885&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21885&range=00-01

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