RFR: 8315934: RISC-V: Disable conservative fences per vendor

[Ludovic Henry]

Conservative fences are not a requirement on some RISC-V hardware for correctness, but can bring a performance penalty. Let's make sure we disable them on a per-vendor basis, and keep them enabled for the default case.

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Commit messages:
 - 8315934: RISC-V: Disable conservative fences per vendor

Changes: https://git.openjdk.org/jdk/pull/15684/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15684&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315934
  Stats: 21 lines in 3 files changed: 17 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15684.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15684/head:pull/15684

PR: https://git.openjdk.org/jdk/pull/15684