[jdk24u] RFR: 8350483: AArch64: turn on signum intrinsics by default on Ampere CPUs

[Patrick Zhang]

Backport the commit to set `-XX:+UseSignumIntrinsic` by default for Ampere CPUs. It is to fix performance problem observed on JMH cases `vm.compiler.Signum|java.lang.*MathBench.sig[nN]um*`. In the worst test cases, run speed is 1~2% of the expected (patched) and functions got severely impacted. So, the fix can be regarded not only a performance fix but also a function problem solving in a manner, which can be a point to support this backport request too.

The backporting is of low risk as the patch is limited to Ampere CPUs only and well verified on Ampere-1A with related jmh and jtreg tier1 tests.

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Commit messages:
 - Backport f529bf712d8946584999dfc98abea60c22c97167

Changes: https://git.openjdk.org/jdk24u/pull/101/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk24u&pr=101&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350483
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk24u/pull/101.diff
  Fetch: git fetch https://git.openjdk.org/jdk24u.git pull/101/head:pull/101

PR: https://git.openjdk.org/jdk24u/pull/101