…tion

i686 already uses SSE2 to do calculations with f32 and f64, but the C
calling convention uses the x87 stack to return values. The Rust calling
convention does not need to do this, and LLVM makes it easy to use XMM0
instead, which saves move instructions and fixes problems with NaN values.

Activating SSE2 with -C target-feature doesn't change the Rust ABI anymore,
while deactivating it still does.