RFR: 8227653: Add VM Global OopStorage

[Kim Barrett]

Please review this change which adds a VMGlobal OopStorage object.  It
is initially being used instead of the conditional JVMCI global storage
object, which is being removed.

Looking for reviewers from all of gc, runtime, and compiler.

To keep things simple for now, this new storage object is (optionally)
included in the processing done by SystemDictionary::oops_do.  Most
existing storage processors use that mechanism.  For most processors,
that's consistent with how JNI global handles are processed.  ZGC uses
a different approach, and provides enough infrastructure that it was
easy to process this new storage object in a way that is consistent
with ZGC's handling of JNI globals.

This change does not attempt to address the problems around changing
the set of OopStorage instance described by JDK-8227054.  This change
was a useful bit of preparation for the work I'm doing on JDK-8227054,
so I split it out as a separate change.

This change also includes a minimal update of Shenandoah, using the
processing of the new storage object by SystemDictionary::oops_do.
It looks like Shenandoah is conceptually similar to ZGC in it's
handling of JNI globals, and should be able to handle this new storage
object similarly, but I'm leaving that to the Shenandoah developers.
You might want to wait for JDK-8227054 though.

Note that neither ZGC nor Shenandoah processed the former conditional
JVMCI global storage.

CR:
https://bugs.openjdk.java.net/browse/JDK-8227653

Webrev:
http://cr.openjdk.java.net/~kbarrett/8227653/open.00/

Testing:
mach5 tier1-5