Refactor sign-extension of signed c-call return values on x86-64:
* Also sign extend short ints (fixes bug reported by Kevin Rosenberg
on sbcl-devel, "FFI size error in sbcl-amd64").
* Move the sign-extension to a :naturalize-gen alien-type-method.
* Remove signed-byte-32 ptype (used only for some sign-extension
hacks, which have now been removed).
* Add some tests.