Avoid referencing undefined ABIs on 64-bit Windows builds

64-bit Windows does not have FFI_STDCALL, FFI_THISCALL, or FFI_FASTCALL.
Fix a warning on 64-bit Windows

When sizeof(size_t) != sizeof(unsigned), adding a size_t to cif->bytes
produces a "possible loss of data" warning.  However, the size_t in
question refers to the size of a single parameter.  Use a cast to avoid
the warning.
Merge pull request #110 from joshtriplett/w64

Fix 64-bit Windows support