RFR: 8369685: RISC-V: refactor code related to RVFeatureValue::enabled [v4]

[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

Hamlin Li has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 19 commits:

 - merge master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - use DEFAULT_VALUE for RVExtFeatureValue::value()
 - review
 - minor
 - minor
 - initial commit
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - ... and 9 more: https://git.openjdk.org/jdk/compare/95380e1e...6dd4b73f

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

Changes: https://git.openjdk.org/jdk/pull/27771/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27771&range=03
  Stats: 46 lines in 2 files changed: 9 ins; 24 del; 13 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