[jdk8u-dev] RFR: 8186902: jcmd GC.run should not be blocked by DisableExplicitGC

[Zhengyu Gu]

I would like to backport this parity bug to openjdk8u.

Original bug: https://bugs.openjdk.java.net/browse/JDK-8186902
Original patch: 
[http://hg.openjdk.java.net/jdk10/jdk10/hotspot/rev/3d1150c7899c](https://hg.openjdk.java.net/jdk10/jdk10/hotspot/rev/3d1150c7899c)


The original patch does not apply cleanly. Besides line shifts, 8u does 
not define `GCCause::_dcmd_gc_run`, but `GCCause::_java_lang_system_gc` instead.

Original code review thread: https://mail.openjdk.java.net/pipermail/jdk8u-dev/2021-September/014288.html

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Commit messages:
 - 8186902: jcmd GC.run should not be blocked by DisableExplicitGC

Changes: https://git.openjdk.java.net/jdk8u-dev/pull/40/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk8u-dev&pr=40&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8186902
  Stats: 5 lines in 1 file changed: 0 ins; 4 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk8u-dev/pull/40.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk8u-dev pull/40/head:pull/40

PR: https://git.openjdk.java.net/jdk8u-dev/pull/40