Fix for PowerPC: handle libffi's integer return type promotion

[yallop]

When the return type of a function is an integer type narrower than the system register size, libffi promotes it to ffi_arg.

On big-endian systems, this causes problems with calls to functions that return small integers (char, short, etc.), since libffi writes the return value to an ffi_arg-sized slot, and ctypes attempts to read the value from the same address using a pointer to the return type rather than a pointer to ffi_arg.

This pull request adds a compensating offset to the return value address passed to ctypes on big-endian systems for both regular function calls (d00922e) and callbacks (d6c200a), and a new test for callbacks that return small integers (9f3cb97).