RFR: 8299375: [PPC64] GetStackTraceSuspendedStressTest tries to deoptimize frame with invalid fp

[Richard Reingruber]

Mark a frame as not fully initialized when copying it from a continuation StackChunk to the stack until the callers_sp (aka back link) is set.

This avoids the assertion given in the bug report when the copied frame is deoptimized before it is fully initialized.
IMHO the deoptimization at that point is a little questionable but it actually only changes the pc of the frame which can be done.
Note that the frame can get extended later (and metadata can get overridden) but [there is code that handles this](https://github.com/openjdk/jdk/blob/34a92466a615415b76c8cb6010ff7e6e1a1d63b4/src/hotspot/share/runtime/continuationFreezeThaw.cpp#L2108-L2110).

Testing: jdk_loom. The fix passed our CI testing. This includes most JCK and JTREG tiers 1-4, also in Xcomp mode, on the standard platforms and also on ppc64le.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/12941/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12941&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299375
  Stats: 15 lines in 3 files changed: 14 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12941.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12941/head:pull/12941

PR: https://git.openjdk.org/jdk/pull/12941