RFR: 8301819: Enable continuations code by default

[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

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

Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/12459/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12459&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301819
  Stats: 4 lines in 2 files changed: 0 ins; 1 del; 3 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