RFR: 8305670: Performance regression in LockSupport.unpark with lots of idle threads [v8]

[Daniel D. Daugherty]

> Address the performance regression in `LockSupport.unpark()` with the following changes:
> - Add `FastThreadsListHandle` helper class to threadSMR.[ch]pp for quickly determining if a JavaThread* is protected by the ThreadsListHandle embedded in the new helper object.
> - Update `Unsafe_Unpark()` to pass `java_lang_Thread::thread_acquire(thread_oop)` to a new instance of the `FastThreadsListHandle` object and only do the unpark work if the target JavaThread* is protected.

Daniel D. Daugherty has updated the pull request incrementally with one additional commit since the last revision:

  dholmes CR - FastThreadsListHandle ctr should use java_lang_Thread::thread() instead of java_lang_Thread::thread_acquire(); clarify comments.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13519/files
  - new: https://git.openjdk.org/jdk/pull/13519/files/d65ab8e4..a076e808

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13519&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13519&range=06-07

  Stats: 9 lines in 3 files changed: 5 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/13519.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13519/head:pull/13519

PR: https://git.openjdk.org/jdk/pull/13519