RFR: 8314480: Memory ordering spec updates in java.lang.ref [v6]

[Brent Christian]

> Classes in the `java.lang.ref` package would benefit from an update to bring the spec in line with how the VM already behaves. The changes would focus on _happens-before_ edges at some key points during reference processing.
> 
> A couple key things we want to be able to say are:
> - `Reference.reachabilityFence(x)` _happens-before_ reference processing occurs for 'x'.
> - `Cleaner.register()` _happens-before_ the Cleaner thread runs the registered cleaning action.
> 
> This will bring Cleaner in line (or close) with the memory visibility guarantees made for finalizers in [JLS 17.4.5](https://docs.oracle.com/javase/specs/jls/se18/html/jls-17.html#jls-17.4.5):
> _"There is a happens-before edge from the end of a constructor of an object to the start of a finalizer (§12.6) for that object."_

Brent Christian has updated the pull request incrementally with one additional commit since the last revision:

  Tweak Reference.enqueue memory consistency effects wording

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/16644/files
  - new: https://git.openjdk.org/jdk/pull/16644/files/9b4b1150..3647f64a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16644&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16644&range=04-05

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/16644.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16644/head:pull/16644

PR: https://git.openjdk.org/jdk/pull/16644