Rollup merge of #148052 - tgross35:stabilize-const_mul_add, r=RalfJung

Stabilize `const_mul_add`

Newly stable API:

```rust
impl {f32, f64} {
    pub const fn mul_add(self, a: Self, b: Self) -> Self;
}
```

This includes making the intrinsics `fmaf{16,32,64,128}` const stable for indirect use, matching similar intrinsics.

Closes: rust-lang/rust#146724

Author: matthiaskrgr