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

[Fei Yang]

On Tue, 12 Sep 2023 14:27:40 GMT, Ludovic Henry <luhenry at openjdk.org> wrote:

> 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.

LGTM. Let's keep the current shape.

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Marked as reviewed by fyang (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/15684#pullrequestreview-1624572418