RFR: 8352218: RISC-V: Zvfh requires RVV

[Robbin Ehn]

Hi please consider.

Added case to turn off UseZvfh when no RVV. 
Which is the cause of the test issues, zvfh on but no rvv.

Also made all case identical and added no warning when default.
Move them to the common init, as the "UseExtension" is not C2 specific.

Manual tested and some random compiler tests.

Thanks, Robbin

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Commit messages:
 - Moved to common
 - Disable UseZvfh when no RVV

Changes: https://git.openjdk.org/jdk/pull/24094/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24094&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8352218
  Stats: 51 lines in 1 file changed: 32 ins; 19 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/24094.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24094/head:pull/24094

PR: https://git.openjdk.org/jdk/pull/24094