Add some SSE _mm_cvt* instructions (#136)

* Add single output _mm_cvt[t]ss_* variants

The *_pi variants are currently blocked by
https://github.com/rust-lang-nursery/stdsimd/issues/74

* Add _mm_cvtsi*_ss

The _mm_cvtpi*_ps intrinsics are blocked by
https://github.com/rust-lang-nursery/stdsimd/issues/74

* Fix Linux builds

Also the si64 variants are only available on x86_64