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

[Patrick Zhang]

Set -XX:+UseSignumIntrinsic by default for Ampere CPUs. It is to fix performance problem found on JMH cases `vm.compiler.Signum|java.lang.*MathBench.sig[nN]um*` where fmov is used to transmit data between GPRs and FPRs with significant time cost.

Verified on Ampere-1A and found the scores (thrpt, ops/s) of `java.lang.*MathBench.sig[nN]um*` improved 40~50%, while `vm.compiler.Signum._1_signumFloatTest` and `vm.compiler.Signum._3_signumDoubleTest` results gained exponential increases. Also passed GHA sanity checks, and Jtreg tier1 on Ampere-1A as function-wise smoke tests.

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Commit messages:
 - 8350483: AArch64: turn on signum intrinsics by default on Ampere CPUs

Changes: https://git.openjdk.org/jdk/pull/23735/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23735&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/jdk/pull/23735.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23735/head:pull/23735

PR: https://git.openjdk.org/jdk/pull/23735