RFR: 8340657: [PPC64] SA determines wrong unextendedSP

[Martin Doerr]

Using the FP as raw_unextendedSP is wrong and causes problems like [JDK-8339772](https://bugs.openjdk.org/browse/JDK-8339772).
"test/hotspot/jtreg/serviceability" tests have passed with this PR.

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Commit messages:
 - 8340657: [PPC64] SA determines wrong unextendedSP

Changes: https://git.openjdk.org/jdk/pull/21139/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21139&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/jdk/pull/21139.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21139/head:pull/21139

PR: https://git.openjdk.org/jdk/pull/21139