RFR: 8284811: Shrink critical section of JNICritical_lock

Wed Apr 13 08:36:45 UTC 2022

This refactoring around gc-locker consists of two commits:

1. Remove `_debug_jni_lock_count` because `verify_critical_count` already verifies #threads in critical regions. `_debug_jni_lock_count` doesn't introduce anything new.

2. Shrink the critical section associated with `JNICritical_lock` to accesses to `_needs_gc` and nothing else.

Test: tier1-6

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Commit messages:
 - finer cr
 - remove _debug_jni_lock_count

Changes: https://git.openjdk.java.net/jdk/pull/8218/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8218&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284811
  Stats: 72 lines in 3 files changed: 5 ins; 47 del; 20 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8218.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8218/head:pull/8218

PR: https://git.openjdk.java.net/jdk/pull/8218