Make constructors `const fn` where possible

I have a library which wraps SIMD types and it would be a nice feature if users were able to create const values with my wrapper types. There's currently no way of doing this in stable Rust.

It would be convenient if some constructor functions could be made const fn so we can create SIMD constants.

For example this is not currently possible:

const ONES : __m128 = unsafe { _mm_set1_ps(1.0) };

Internally functions like _mm_set1_ps are quite straight forward:

pub unsafe fn _mm_set1_ps(a: f32) -> __m128 {
  __m128(a, a, a, a)
}

__m128 is a repr(simd) tuple struct.

I would propose making all of the _mm_set* functions const fn and any other construction functions in stdarch for that matter.

Start by locating the _mm_set* constructors and the __m128 repr(simd) tuple struct in stdarch. Check which construction functions can be declared const fn, then verify that the issue's const ONES example can create a SIMD constant on stable Rust. Done means the applicable constructors support const evaluation without changing unrelated APIs.