[jdk21u-dev] RFR: 8322962: Upcall stub might go undetected when freezing frames

[Aleksey Shipilev]

Clean backport to fix Virtual Threads and FFM interactions.

Additional testing:
 - [x] New regression testing passes before and after the fix (expected, since in JDK 21 `VThreadPinner` does not use FFM)
 - [x] MacOS AArch64 server fastdebug, `jdk_loom hotspot_loom`
 - [ ] Linux x86_64 server fastdebug, `all`

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Commit messages:
 - Backport 93feda3d9a1807422c7f47703358aabd2e8639b8

Changes: https://git.openjdk.org/jdk21u-dev/pull/532/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=532&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322962
  Stats: 34 lines in 6 files changed: 21 ins; 4 del; 9 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/532.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/532/head:pull/532

PR: https://git.openjdk.org/jdk21u-dev/pull/532