[foreign] RFR: struct/global functions stubs should attached to some scope

[Maurizio Cimadamore]

Hi,
there is a currently a problem with function pointers: when you assign a 
functional interface object to a struct/global variable, a stub is 
generated on the fly; the native stub has a weak link back to the 
UpcallHandler object it originated from - but the UpcallHandler object 
is not kept anywhere - which means the GC is free to collect it as soon 
as possible (and this will also cause deallocation of the native stub, 
via the cleaner).

We need some way to attach stub resources to scopes (either the scope of 
a resource or the recently-added global library scope). That's what this 
patch does. When Scope::close is called, the list of stub is null-ed, so 
that the GC can deallocate things as expected.

Note that this is an incremental improvement while we await for a more 
cohesive native resource management strategy/API.

The test for function callbacks has been tweaked to feature some 
System.gc - w/o this patch, these alterations cause the test to fail 
pretty reliably.

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/func-scope/

Maurizio