RFR: 8228748: Remove GCLocker::_doing_gc

[Kim Barrett]

On Wed, 17 Feb 2021 08:00:04 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

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

Looks good.  Copyright for gcLocker.hpp needs to be updated.

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Marked as reviewed by kbarrett (Reviewer).

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