RFR: ZGC: Add support for JFR leak profiler

[erik.osterlund at oracle.com]

Hi,

The JFR leak profiler is currently not supported on ZGC, because it has 
not been accessorized appropriately.
This patch adds the required Access API sprinkling necessary, to enable 
this for ZGC. I did the following:

1) Renamed UnifiedOop to UnifiedOopRef, because it describes the 
reference to an oop, not the oop itself.
2) Changed UnifiedOopRef to be a POD, instead of an AllStatic that 
passes around its encoded data as
    const oop* (even though it might be a tagged pointer to a narrow 
oop), improving type safty.
3) Added another tag bit to UnifiedOopRef, allowing it to keep track of 
whether the described location is
    IN_NATIVE or IN_HEAP. The dereference function performs the 
appropriate NativeAccess/HeapAccess with the
    AS_NO_KEEPALIVE decotator, which is fine because the leak profiler 
does not create any edges to oops
    found during traversal.
4) Removed code blob oop iteration, because it can't be supported 
appropriately by the current UnifiedOopRef
    mechanism (because oops are misaligned, and UnifiedOopRef requires 
tag bits - I believe this has never
    worked properly)
6) Accessorized loads on strong roots to 
NativeAccess<AS_NO_KEEPALIVE>::oop_load. JFR leak profiler never
    creates new references to oops that are visited, so it does not need 
to keep things alive
7) Explicitly told the oop iteration closures to not visit referents. 
The default referent strategy used
    before is DO_DISCOVERY, which doesn't seem right.
8) Fixed a pre-existing issue where the array index reported in the leak 
chain was calculated incorrectly
    as the byte offset from the array base. It should be scaled by the 
element length of the array.

Ran some JFR leak profiler tests, and some ad-hoc leak profiling in 
various applications. Also checked that
there were no observable perf regressions in the iterations, and there 
were not.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8235174

Webrev:
http://cr.openjdk.java.net/~eosterlund/8235174/webrev.00/

Thanks,
/Erik