RFR: 8284161: Implementation of Virtual Threads (Preview) [v9]

[Alan Bateman]

> This is the implementation of JEP 425: Virtual Threads (Preview).
> 
> We will refresh this PR periodically to pick up changes and fixes from the loom repo.
> 
> Most of the new mechanisms in the HotSpot VM are disabled by default and require running with `--enable-preview` to enable.
> 
> The patch has support for x64 and aarch64 on the usual operating systems (Linux, macOS, and Windows). There are stubs (calling _Unimplemented_) for zero and some of the other ports. Additional ports can be contributed via PRs against the fibers branch in the loom repo.
> 
> There are changes in many areas. To reduce notifications/mails, the labels have been trimmed down for now to hotspot, serviceability and core-libs. We can add additional labels, if required, as the PR progresses.
> 
> The changes include a refresh of java.util.concurrent and ForkJoinPool from Doug Lea's CVS. These changes will probably be proposed and integrated in advance of this PR.
> 
> The changes include some non-exposed and low-level infrastructure to support the (in draft) JEPs for Structured Concurrency and Extent Locals. This is to make life a bit easier and avoid having to separate VM changes and juggle branches at this time.

Alan Bateman has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 13 commits:

 - Refresh ee9fa8ed05ec22de7a13383052d68aa8aa7832ec
 - Merge with jdk-19+20
 - Refresh 7965cc6b168e567ac2596f2fbc3b00a7d99b7e1e
 - Refresh 8d8f0a2fd646e57fe6b4e8ab669f836dc46dda69
 - Refresh cf561073f48fad58e931a5285b92629aa83c9bd1
 - Merge with jdk-19+19
 - Refresh
 - Refresh
 - Refresh
 - Refresh
 - ... and 3 more: https://git.openjdk.java.net/jdk/compare/16a8ebbf...51bc652d

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Changes: https://git.openjdk.java.net/jdk/pull/8166/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8166&range=08
  Stats: 103888 lines in 1143 files changed: 93996 ins; 4227 del; 5665 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8166.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8166/head:pull/8166

PR: https://git.openjdk.java.net/jdk/pull/8166