Function safe_arch::set_splat_m256d

#[must_use]pub fn set_splat_m256d(f: f64) -> m256d

This is supported with target feature avx only.

Splat an f64 arg into an m256d lane.

let a = set_splat_m256d(56.0).to_array();
assert_eq!(a, [56.0; 4]);