[foreign] RFR 8215553: Remove SystemABI::freeUpcallStub

[Maurizio Cimadamore]

Hi,
few months ago we moved to a new model for managing lifecycle of 
callbacks, where callbacks were explicitly allocated through a Scope; 
when the scope is closed, all callbacks registered onto that scope are 
freed.

Back then, I failed to realize that we don't really need an API point to 
free the callback - we can just rely on a Cleaner to do its job when the 
callback is no longer referenced from Java code. Since a callback will 
always be stored in some scope (when active), this will effectively 
prevent it from being collected.

Note: this model is very different from what we had before, where 
callbacks were NOT allocated using a Scope, but only implicitly by 
passing lambdas to functions. In that case, using a Cleaner was bad, as 
it  sometimes led to GC happening _in the middle_ of a function call. 
This can no longer happen, as a Scope will now hold tight on the 
callbacks used at any given point in time.

Of course, users of the low-level SystemABI, must pay extra care: as 
soon as an upcall symbol goes out of scope, it is ready for collection; 
that means that if a client of SystemABI wants an upcall symbol to 
remain alive, it has to stash it somewhere. Given the audience of this 
SystemABI API, I think this is an acceptable compromise (and one I've 
seen elsewhere, e.g. JNR). On the plus side, the API got tighter, as we 
no longer need a 'free' method.

Webrev:
http://cr.openjdk.java.net/~mcimadamore/panama/8215553/

Cheers
Maurizio