[foreign] RFR: add test for mutually referring structs

[Maurizio Cimadamore]

Hi,
as part of some worek I'm doing on unresolved layout, I thought it would 
be a good idea to start checking in some tests stressing the unresolved 
layout resolution machinery, so that, if we make changes in this area, 
we make sure we do not regress.

This test checks two mutually referring structs; there are two flavor of 
the test, one where the structs refer to each other with a pointer, 
another where the structs refer to themselves via a double pointer.

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/mutual-structs/

Maurizio