[8u] 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


8u Webrev: http://cr.openjdk.java.net/~zgu/JDK-8186902-8u/webrev.00/

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.


Thanks,

-Zhengyu