RFR: 8315338: RISC-V: Change flags for stable extensions to non-experimental

[Robbin Ehn]

Hi all, please consider.

These flags can be automatically turned on if you are using a 6.5 kernel with the proper hardware.
So they are not considered experimental.

Running tier1

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Commit messages:
 - Flags

Changes: https://git.openjdk.org/jdk/pull/15585/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15585&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315338
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/15585.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15585/head:pull/15585

PR: https://git.openjdk.org/jdk/pull/15585