[jdk19] RFR: 8290004: [PPC64] JfrGetCallTrace: assert(_pc != nullptr) failed: must have PC

[Martin Doerr]

We must read last_Java_sp first memory ordering wise. Otherwise, last_Java_pc can be 0.
Note: Writer side already uses release semantics, see javaFrameAnchor_ppc.hpp.

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Commit messages:
 - 8290004: [PPC64] JfrGetCallTrace: assert(_pc != nullptr) failed: must have PC

Changes: https://git.openjdk.org/jdk19/pull/124/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk19&pr=124&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290004
  Stats: 15 lines in 2 files changed: 0 ins; 13 del; 2 mod
  Patch: https://git.openjdk.org/jdk19/pull/124.diff
  Fetch: git fetch https://git.openjdk.org/jdk19 pull/124/head:pull/124

PR: https://git.openjdk.org/jdk19/pull/124