RFR: 8301819: Enable continuations code by default [v3]

[Alan Bateman]

> The continuations code added in JDK 19 with JEP 425 is currently disabled by default and is only enabled when running with preview features enabled (`--enable-preview`). Disabled by default was the pragmatic choice at that time to reduce risk and possible performance regressions. We'd like to change this so the code is enabled by default on the platforms that have a port. Enabling this code by default helps to shake out any issues early in JDK 21 and sets things up to make virtual threads a permanent feature in the future. If something comes up in the next few weeks/months then it can be changed to be disabled by default again. This change has no impact on the builds that do not have a port.
> 
> Testing: tier1-tier6

Alan Bateman has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - Remove include
 - Merge
 - Merge
 - Update copyright date
 - Initial commit

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12459/files
  - new: https://git.openjdk.org/jdk/pull/12459/files/38f9ccde..df807cb1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12459&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12459&range=01-02

  Stats: 1889 lines in 90 files changed: 1665 ins; 77 del; 147 mod
  Patch: https://git.openjdk.org/jdk/pull/12459.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12459/head:pull/12459

PR: https://git.openjdk.org/jdk/pull/12459