[8u] RFA 8044107 : Add Diagnostic Command to list all ClassLoaders

[David Buck]

Hi!

Please approve the push of this backport to 8u-dev. The change has been 
reviewed and the necessary enhancement approval has been obtained.

bug report:
https://bugs.openjdk.java.net/browse/JDK-8044107

JDK 8 backport code review:
http://mail.openjdk.java.net/pipermail/hotspot-dev/2017-December/029655.html

enhancement backport approval:
http://mail.openjdk.java.net/pipermail/jdk8u-dev/2017-December/007145.html

Cheers,
-Buck