RFR: 8287437: Temporarily disable Continuations::enabled() for platforms which don't have an implementation, yet [v2]

[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.

Martin Doerr has updated the pull request incrementally with one additional commit since the last revision:

  Invert check: Platforms which do support Loom

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8919/files
  - new: https://git.openjdk.java.net/jdk/pull/8919/files/64556dda..d9084e43

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8919&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8919&range=00-01

  Stats: 5 lines in 1 file changed: 2 ins; 2 del; 1 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