RFR: 8137018: [JVMCI] Encapsulate new Thread fields for JVMCI

[Tom Rodriguez]

This evacuates all JVMCI related methods and fields into a separately declared struct.

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Commit messages:
 - 8137018: [JVMCI] Encapsulate new Thread fields for JVMCI

Changes: https://git.openjdk.java.net/jdk/pull/5339/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5339&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8137018
  Stats: 547 lines in 16 files changed: 282 ins; 207 del; 58 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5339.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5339/head:pull/5339

PR: https://git.openjdk.java.net/jdk/pull/5339