RFR (S): 8191894: Refactor weak references in JvmtiTagHashmap to use the Access API

[Erik Österlund]

Hi,

The JVMTI tag hashmap has weak oop references that are handled using raw 
oop accesses and a G1-specific SATB enqueue call when leaking out 
objects from the tag map, requiring them to be marked as live by G1.

This should now be refactored to use the Access API to annotate that 
these are ON_PHANTOM_OOP_REF, and refactor the raw oop loads to use 
ON_PHANTOM_OOP_REF | AS_NO_KEEPALIVE.

Webrev:
http://cr.openjdk.java.net/~eosterlund/8191894/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8191894

Thanks,
/Erik