RFR: 8273912: Add threadControl_dumpThread(jthread) function

[Chris Plummer]

Trivial change to add an internal API that is useful when debugging. Only included in debug builds, and has no impact on debug agent functionality.

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

Commit messages:
 - Add threadControl_dumpThread()

Changes: https://git.openjdk.java.net/jdk/pull/5554/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5554&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273912
  Stats: 12 lines in 2 files changed: 12 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5554.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5554/head:pull/5554

PR: https://git.openjdk.java.net/jdk/pull/5554