Auto merge of #32066 - ruud-v-a:fma, r=alexcrichton

This adds support for fused multiply-add and multiply-subtract vector intrinsics for 128 and 256-bit vectors of `f32` and `f64`. These correspond to the intrinsics [listed here](https://software.intel.com/en-us/node/523929) except for the `_ss` and `_sd` variants. The intrinsics added are:

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

The “fma” target feature must be enabled by passing `-C target-feature=+fma` to rustc when using these, otherwise LLVM will complain.

I verified locally that the `x86_mm256_fmadd_ps` and `x86_mm256_fmsub_ps` work.