RFR: 8331735: UpcallLinker::on_exit races with GC when copying frame anchor

[Jorn Vernee]

There is a subtle race in `UpcallLinker::on_exit` between copying of the old frame anchor back into place, and the GC. Since this copy is not atomic, it may briefly appear as if a thread has no last Java frame, while still in the `_thread_in_native` state, which leads to the GC skipping processing of any active Java frames.

This code was originally adapted from `JavaCallWrapper::!JavaCallWrapper` - the JNI mechanism for upcalls - but in that case the frame anchor copy happens in the `_thread_in_vm` state, which means the GC will wait for the thread to get to a safepoint.

The solution proposed here is to do the frame anchor copy in the java thread state, before transitioning  back to the native state. The java thread state, like the vm thread state, is also 'safe' i.e. the GC will wait for the thread to get to a safepoint, so we can safely do our non-atomic copy of the frame anchor.

Additionally, this PR resolves a similar issue in `on_entry`, by moving the clearing of the pending exception (in case native code use a JNI API and didn't handle the exception afterwards). We now also skip checking for async exceptions when transitioning from native to java, so we don't immediately clear them. Any async exceptions will be picked up at the next safepoint instead.

Special thanks to @stefank and @fisk for finding the root cause, and @jaikiran for testing and debugging.

Testing: tier 1-4, 20k runs of the failing test on linux-aarch64.

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Commit messages:
 - Merge branch 'master' into SafeFrameAnchor
 - Merge branch 'master' into SafeFrameAnchor
 - Don't touch frame anchor or current exception oop in native state

Changes: https://git.openjdk.org/jdk/pull/21742/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21742&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331735
  Stats: 15 lines in 1 file changed: 5 ins; 9 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21742.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21742/head:pull/21742

PR: https://git.openjdk.org/jdk/pull/21742