[jdk21u-dev] 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 **defect** fixing in a manner, which can be a point to support this backport request too. 

The commit is in jdk mainline and got successfully merged to jdk24u. It is of low risk as the patch is limited to Ampere CPUs only and well tested on Ampere-1A with related jmh and jtreg tier1 tests.

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Commit messages:
 - Backport 23389d08a2b5c0840d2b18fb2e74d2d112b3192d

Changes: https://git.openjdk.org/jdk21u-dev/pull/1606/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1606&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/jdk21u-dev/pull/1606.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1606/head:pull/1606

PR: https://git.openjdk.org/jdk21u-dev/pull/1606