RFR: 8354572: Turn off AlwaysMergeDMB for Ampere CPU by default

[Aleksey Shipilev]

On Tue, 15 Apr 2025 08:16:08 GMT, Liming Liu <lliu at openjdk.org> wrote:

> Hi all, I found that "dmb ishst + dmb ishld" is obviously faster than "dmb ish" on Ampere CPU, so I submit this patch to make the former be default.

So basically the similar performance behavior as with Neoverse cores, which already opts in. Looks fine.

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Marked as reviewed by shade (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/24652#pullrequestreview-2767357649