[jdk21u-dev] RFR: 8350106: [PPC] Avoid ticks_unknown_not_Java AsyncGetCallTrace() if JavaFrameAnchor::_last_Java_pc not set

[Richard Reingruber]

This pull request contains a backport of commit [030c85de](https://github.com/openjdk/jdk/commit/030c85de1376123615e804f98084cb3723205819) from the [openjdk/jdk](https://git.openjdk.org/jdk) repository.

The commit being backported was authored by Richard Reingruber on 11 Mar 2025 and was reviewed by Martin Doerr.

It does not apply because of copyright header differences and because `frame::kind` doesn't exist yet in jdk21u but appears in the context of the changes.

Risk is low because the patch is small and ppc only. The code is only reached when profiling with jfr or async-profiler.

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Commit messages:
 - Backport 030c85de1376123615e804f98084cb3723205819

Changes: https://git.openjdk.org/jdk21u-dev/pull/1971/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1971&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350106
  Stats: 27 lines in 2 files changed: 17 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1971.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1971/head:pull/1971

PR: https://git.openjdk.org/jdk21u-dev/pull/1971