[jdk23u] RFR: 8340657: [PPC64] SA determines wrong unextendedSP

[Martin Doerr]

Clean backport of [JDK-8340657](https://bugs.openjdk.org/browse/JDK-8340657).

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Commit messages:
 - Backport 49d15edd31c863faf3722af1bae8b50662ecf71f

Changes: https://git.openjdk.org/jdk23u/pull/125/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk23u&pr=125&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340657
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk23u/pull/125.diff
  Fetch: git fetch https://git.openjdk.org/jdk23u.git pull/125/head:pull/125

PR: https://git.openjdk.org/jdk23u/pull/125