RFR: 8287437: Temporarily disable Continuations::enabled() for platforms which don't have an implementation, yet

[Martin Doerr]

Hello,

I'd like to disable `Continuations::enabled()` for platforms, which don't support Loom, yet. This enables working on Panama which is also controlled by `--enable-preview`.

I'm proposing this for all platforms which have an unresolved porting issue here:
[JDK-8284161](https://bugs.openjdk.java.net/browse/JDK-8284161)

Please review and provide feedback if this makes sense for your platform.

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Commit messages:
 - 8287437: Temporarily disable Continuations::enabled() for platforms which don't have an implementation, yet

Changes: https://git.openjdk.java.net/jdk/pull/8919/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8919&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287437
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8919.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8919/head:pull/8919

PR: https://git.openjdk.java.net/jdk/pull/8919