function: Correctly convert from ffi return values to GIArgument on big-endian

ffi always returns a long value even if a function's return type is
e.g. guint8.  We can't just directly give ffi_call a GIArgument to
store its return value because when the size of the return type is
smaller than long, it will write into the wrong address.

Instead cast the value at the C level.

Based on a patch from Ray Strode <rstrode@redhat.com>

https://bugzilla.gnome.org/show_bug.cgi?id=665152