[Go] Ensure structs are properly packed between 8c and GCC/clang

Ensure SWIG generated structs are properly packed.
Without this fix, crashes are seen on windows/386 when calling functions with int64 arguments.

It also mimics what CGO itself [1].

[1] https://code.google.com/p/go/source/browse/src/cmd/cgo/out.go?r=22b0839c6cd425dc61f20c34977bc85fb3d7d475#614