RFR: 8289231: Move java_lang_VirtualThread to continuationJavaClasses.hpp

[Ioi Lam]

This is a follow up to [JDK-8288623](https://bugs.openjdk.org/browse/JDK-8288623). `java_lang_VirtualThread` is part of loom and should be move to continuationJavaClasses.hpp, which contains the accessors for the other java classes used by loom.

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Commit messages:
 - 8289231: Move java_lang_VirtualThread to continuationJavaClasses.hpp

Changes: https://git.openjdk.org/jdk/pull/9298/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9298&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289231
  Stats: 303 lines in 15 files changed: 154 ins; 147 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/9298.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9298/head:pull/9298

PR: https://git.openjdk.org/jdk/pull/9298