[jdk17u-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*`.  For example, `vm.compiler.Signum._1_signumFloatTest` thrpt score becomes 30x better on both jdk mainline and jdk17u-dev. The backporting can be very safe as it 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/jdk17u-dev/pull/3300/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=3300&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/jdk17u-dev/pull/3300.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/3300/head:pull/3300

PR: https://git.openjdk.org/jdk17u-dev/pull/3300