RFR 8237351: Add method to SystemABI for explicitly freeing upcall stubs

[Jorn Vernee]

Hi,

Please review the following:

Bug: https://bugs.openjdk.java.net/browse/JDK-8237351
Webrev: 
http://cr.openjdk.java.net/~jvernee/panama/webrevs/8237351/webrev.00/

This adds a freeUpcallStub method to SystemABI for explicitly freeing an 
upcall stub, and uses it in some of the existing tests with a mix of 
Cleaner and explicit calls.

I slightly changed the native implementation to bail out when it sees 
that we are trying to pass an address that is not a stub (previously 
this was just an assert), and now returns a boolean that indicates 
whether freeing succeeded, so that we can throw an exception when it 
doesn't (for instance in the case that a random address is passed).

Thanks,
Jorn