RFR: 8271490: [ppc] [s390]: Crash in JavaThread::pd_get_top_frame_for_profiling

[Lutz Schmidt]

Please review this pull request. It hardens the code on ppc and s390 against crashes while collecting stack traces for JFR. Stack frame collection in prims/forte.cpp will benefit from this fix as well as it uses the same platform-specific code.

Testing:
SAP-internal functional and stress testing. Verifying a positive effect is hard because the targeted crashes occur only very rarely. No negative side effects of the fix have been observed.

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Commit messages:
 - 8271490: [ppc] [s390]: Crash in JavaThread::pd_get_top_frame_for_profiling

Changes: https://git.openjdk.java.net/jdk/pull/5071/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5071&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271490
  Stats: 105 lines in 4 files changed: 76 ins; 12 del; 17 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5071.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5071/head:pull/5071

PR: https://git.openjdk.java.net/jdk/pull/5071