RFR: 8369685: RISC-V: refactor code related to RVFeatureValue::enabled

[Hamlin Li]

Hi,
Can you help to review this patch?
@RealFYang 

Original discussion is here: https://github.com/openjdk/jdk/pull/27562#discussion_r2415984981.
Although seems we can not remove it, but we can still remove some of the redundant check like `mvendorid.enabled()`.
And also do some simple refactoring related to enable/disable/enabled and so on.

Thanks

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

Commit messages:
 - minor
 - minor
 - initial commit
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - ... and 4 more: https://git.openjdk.org/jdk/compare/28460ca3...48c78ad9

Changes: https://git.openjdk.org/jdk/pull/27771/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27771&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369685
  Stats: 41 lines in 2 files changed: 10 ins; 23 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/27771.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27771/head:pull/27771

PR: https://git.openjdk.org/jdk/pull/27771