RFR: JDK-8212186: JVMTI lacks a few GC barriers/hooks

[Roman Kennke]

Erik recently pointed out that  JvmtiTagHashmapEntry::equals() is
lacking on obj-equals-barrier, and that JvmtiTagHashmap::hash() better
does ::resolve() the object to a stable value before casting it to a raw
address.

This lead us to do some more JVMTI testing and we found that we have a
little inconsistency with reporting object size too: the VMObjectAlloc
callback now reports a different size that GetObjectSize(). This is also
fixed by this proposed change:

Bug:
https://bugs.openjdk.java.net/browse/JDK-8212186
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8212186/webrev.00/

Testing: hotspot/tier1, nsk/jvmti with Shenandoah

Ok?

Roman