RFR: 8228748: Remove GCLocker::_doing_gc

[Albert Mingkun Yang]

Some refactoring in `GCLocker` and more comments in `jni_lock` on how the synchronization works there.

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Commit messages:
 - lock

Changes: https://git.openjdk.java.net/jdk/pull/2602/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2602&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8228748
  Stats: 15 lines in 2 files changed: 4 ins; 5 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2602.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2602/head:pull/2602

PR: https://git.openjdk.java.net/jdk/pull/2602