RFR: 8218734: SA: Incorrect and raw loads of OopHandles

[Stefan Karlsson]

Hi all,

Please review this patch to fix the resolving of oops inside the (VM) 
OopHandles.

https://cr.openjdk.java.net/~stefank/8218734/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8218734

Before this patch the OopHandle::_obj was assumed to be located at 
offset 0, and the SA resolved OopHandle (Klass::_java_mirror and 
ClassLoaderData::_class_loader) without the required load barrier for 
ZGC. I've added a new class VMOopHandle (The SA already has a 
OopHandle), which handles the resolving (load or load + barrier).

The OopHandle type is grouped under the Oops section in vmStructs.cpp. 
It's not an oop, so I added a newline to separate it out from the rest. 
Any suggestion of a better location in that file?

Tested with the jtreg tests in serviceability/sa + patches to make ZGC 
usable with the SA's hprof dumping.

Thanks,
StefanK