[foreign] RFR: avoid use of ids for upcall handlers

[Maurizio Cimadamore]

Hi,
I've attempted to replace the use of integer ids to represent the upcall 
handler we wish to call from native code. Instead, this patch stores the 
address of the upcall handler object _inside_ the generate stub (and has 
the stub jump over it). Since the GC can move objects around, what is 
saved in the code buffer is a global jni handle. I also added logic to 
deallocate both the stub and the global handle when the associated 
UpcallStub Java object is no longer used - this never happened before 
since all the UpcallHandler instances were kept alive inside a static 
array list (which is now removed).

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/upcall_cleanup/

Maurizio