Define x86 fused multiply-add intrinsics
Define x86 fused multiply-add intrinsics

This defines the following intrinsics for 128 and 256 bit vectors of f32
and f64:

 * `fmadd`
 * `fmaddsub`
 * `fmsub`
 * `fmsubadd`
 * `fnmadd`
 * `fnmsub`

The `_sd` and `_ss` variants are not included yet.

Intel intrinsic reference: https://software.intel.com/en-us/node/523929

The intrinsics there are listed under AVX2, but in the Intel Intrinsic
Guide they are part of the "FMA" technology, and LLVM puts them under
FMA, not AVX2.

GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust