RFR: 8340796: Use a consistent order when loading cxq and EntryList

[Fredrik Bredberg]

Made sure we use a consistent order when reading ObjectMonitor EntryList and cxq while exiting the monitor.
That consistent order is EntryList before cxq. Added a comment in the shared code to explaining why it's okay that waiters slip undetected between reading EntryList and cxq.

Tested ok tier1-3.

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

Commit messages:
 - 8340796: Use a consistent order when loading cxq and EntryList

Changes: https://git.openjdk.org/jdk/pull/21584/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21584&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340796
  Stats: 28 lines in 2 files changed: 24 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/21584.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21584/head:pull/21584

PR: https://git.openjdk.org/jdk/pull/21584