RFR: 8358013: [PPC64] VSX has poor performance on Power8

[Martin Doerr]

Power8 only has limited VSX instructions for the superword optimization and the Vector API and the performance is bad. Let's only use it on Power9 and newer by default.

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Commit messages:
 - 8358013: [PPC64] VSX has poor performance on Power8

Changes: https://git.openjdk.org/jdk/pull/25514/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25514&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358013
  Stats: 8 lines in 2 files changed: 7 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/25514.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25514/head:pull/25514

PR: https://git.openjdk.org/jdk/pull/25514