Function safe_arch::set_splat_m256

#[must_use]pub fn set_splat_m256(f: f32) -> m256

This is supported with target feature avx only.

Splat an f32 arg into an m256 lane.

let a =
  set_splat_m256(56.0).to_array();
assert_eq!(a, [56.0; 8]);