RFR: 8212184: Incorrect oop ref strength used for referents in FinalReference

[Per Liden]

AccessBarrierSupport::resolve_unknown_oop_ref_strength() returns an 
incorrect oop ref strength for referents in FinalReference objects. It 
currently returns ON_WEAK_OOP_REF when it should return 
ON_STRONG_OOP_REF. This is not really an issue for any GC except ZGC 
when using the ZHeapIterator to walk the heap while resurrection is blocked.

Bug: https://bugs.openjdk.java.net/browse/JDK-8212184
Webrev: http://cr.openjdk.java.net/~pliden/8212184/webrev.0

/Per