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

[Liming Liu]

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.

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Commit messages:
 - Turn off AlwaysMergeDMB for Ampere

Changes: https://git.openjdk.org/jdk/pull/24652/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24652&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8354572
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/24652.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24652/head:pull/24652

PR: https://git.openjdk.org/jdk/pull/24652