Lift upcall sharing mechanism to AbstractLinker

Upcall stubs can share a Java wrapper class if they both have the same function descriptor. The wrapper class is customized for a particular target MethodHandle using MethodHandles.insertArguments.

Lift the entry point for this mechanism to AbstractLinker, where it can live next to the similar mechanism that exists for downcall method handles, and so that it can also be used for the fallback linker (which currently doesn't do any sharing).