RFR: 8276663: Cleanup NMT AccessLock

[Zhengyu Gu]

Please review this cleanup patch to NMT AccessLock:

* Make LockState enum private, no external use.
* const lock location pointer
* Use memory_order_relaxed for lock counter updates.

Test:
- [x] hotspot_nmt on x86_64 and aarch64

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Commit messages:
 - Update copyright year
 - v0

Changes: https://git.openjdk.java.net/jdk/pull/6267/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6267&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276663
  Stats: 11 lines in 2 files changed: 1 ins; 1 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6267.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6267/head:pull/6267

PR: https://git.openjdk.java.net/jdk/pull/6267