RFR: 8347463: jdk/jfr/threading/TestManyVirtualThreads.java crashes with assert(oopDesc::is_oop_or_null(val)) [v9]

[Anton Seoane Ampudia]

> This PR introduces a fix for a intermittent assert crash due to a non-oop found in the stack when deoptimizing.
> 
> The `inline_native_GetEventWriter` JFR intrinsic performs a call into the runtime, which can safepoint, to write a checkpoint for the vthread. This call returns a global handle (`jobject`) that then gets resolved to a raw oop.
> 
> However, the corresponding `jfr_write_checkpoint_Type` does not set any return, modelling the call as `void`. If a safepoint hits in the small window after the stub returns but before the writer oop is used, and the GC moves the object in that window, the deoptimization path cannot resolve a handle that it never recorded, leading to the subsequent crash.
> 
> An IR Framework test is introduced to exercise the error explicitly. Additionally, related documentation in form of comments in the appropriate file (`runtime.hpp`) is added to hopefully prevent similar cases in the future.
> 
> **Testing:** passes tiers 1-5

Anton Seoane Ampudia has updated the pull request incrementally with one additional commit since the last revision:

  Adjust comment

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27913/files
  - new: https://git.openjdk.org/jdk/pull/27913/files/9ff84e73..aa09c1bb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27913&range=08
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27913&range=07-08

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/27913.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27913/head:pull/27913

PR: https://git.openjdk.org/jdk/pull/27913