request for review (M): 4965777 GC changes to support use of discovered field for pending references

[Ramki Ramakrishna]

CR's 4243978 and  4268317 have proposed to use the discovered field of
a java.lang.ref.reference object to link the objects in the pending list.
This requires changes in both GC and in the reference handler code.
The JVM adaptsso as to allow it to run either inside a newer JDK which 
uses the
discovered field to link the references or inside an older JDK which 
uses the
next field for that purpose. Although the JDK part of the changes are
also being submitted for review here, that part will be integrated as a 
separate
CR and changeset into the JDK repo following the integration of the JVM 
changes
into an appropriate version of the HotSpot (express) repos.

   JVM webrev:  http://cr.openjdk.java.net/~ysr/4965777/hotspot/webrev/
   JDK webrev:   http://cr.openjdk.java.net/~ysr/4965777/jdk/webrev/

Many thanks to Mandy Chung and John Coomes for advice and for
earlier reviews; and to Mandy for additional testing help.

Thanks for any other reviews or comments.
-- ramki