RFR: 8137018: [JVMCI] Encapsulate new Thread fields for JVMCI [v2]

[Tom Rodriguez]

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

Tom Rodriguez has updated the pull request incrementally with one additional commit since the last revision:

  Review cleanups

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5339/files
  - new: https://git.openjdk.java.net/jdk/pull/5339/files/472073a7..9af54d4b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5339&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5339&range=00-01

  Stats: 58 lines in 3 files changed: 9 ins; 22 del; 27 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