RFR: 8284811: Shrink critical section of JNICritical_lock [v2]

[Albert Mingkun Yang]

> 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

Albert Mingkun Yang has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  finer cr

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8218/files
  - new: https://git.openjdk.java.net/jdk/pull/8218/files/fff3e11b..99672c0b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8218&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8218&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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