Fix compile errors caused by __m128 and fnstcw on RISC-V

[XieJiSS]

Closes #12

[swesterfeld]

I've merged it now, avoiding m128 outside `#ifdef SSE__` for all platforms (not just RISC-V) as https://github.com/swesterfeld/spectmorph/commit/6fded91ed872a34d27c3b9f0f32dbb6c1549676f

Thanks!