RFR: Export JavaThread::_lock_id, ObjectMonitor::_stack_locker and markWord::marked_value.

[Yudi Zheng]

Essential exports for implementing fibers support in Graal.

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

Commit messages:
 - Export JavaThread::_lock_id, ObjectMonitor::_stack_locker and markWord::marked_value.

Changes: https://git.openjdk.org/loom/pull/211/files
  Webrev: https://webrevs.openjdk.org/?repo=loom&pr=211&range=00
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/loom/pull/211.diff
  Fetch: git fetch https://git.openjdk.org/loom.git pull/211/head:pull/211

PR: https://git.openjdk.org/loom/pull/211