[jdk20u] RFR: 8300575: JVMTI support when using alternative virtual thread implementation [v2]

[Tyler Steele]

> This PR backports modifications to the jvmti code to add support for vthreads. Without this change, we see many failures in our internal testing.
> 
> These changes did not apply cleanly in several places because of changes related to JDK-8299837 (Replace NULL with nullptr in hotspot code and tests). In these places, the changes expected code containing nullptr, but found NULL. In these spots, kept the code from the backported change, but changed nullptr back to NULL for consistency with the rest of the code base.
> 
> Testing is underway.

Tyler Steele has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains one commit:

  8300575: JVMTI support when using alternative virtual thread implementation
  
  Reviewed-by: lmesnik, sspitsyn, alanb

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Changes: https://git.openjdk.org/jdk20u/pull/60/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk20u&pr=60&range=01
  Stats: 635 lines in 69 files changed: 514 ins; 77 del; 44 mod
  Patch: https://git.openjdk.org/jdk20u/pull/60.diff
  Fetch: git fetch https://git.openjdk.org/jdk20u.git pull/60/head:pull/60

PR: https://git.openjdk.org/jdk20u/pull/60