[BUG][FIX] Linux 2GB problem in FFI

This bug is similar to the image segment loading problem when the
object memory spans the 2GB boundary. Ned has fixed that, but FFI uses
the #isInMemory: method which was not fixed.

The attached changeset makes #startOfMemory return an unsigned value
(just like endOfMemory, which Ned fixed), and casts the parameter of
#isInMemory: to unsigned.

- Bert -