Add Atomic*::get_mut_slice

[WaffleLapkin]

This PR adds the inverse of Atomic*::from_mut_slice introduced in #94384 with the following API:

// core::sync::atomic

impl Atomic* {
    fn get_mut_slice(this: &mut [Self]) -> &mut [*];
}

cc @cuviper

---

For now I've used the same tracking issue as Atomic*::from_mut_slice, should I open a new one?

[rust-highfive]

r? @Mark-Simulacrum

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[cuviper]

I don't think you need the alignment cfg for this direction, just as get_mut does not.

[WaffleLapkin]

Ah, I didn't notice that align_of<Atomic*>() >= align_of::<*>() always holds 😅

[Mark-Simulacrum]

@bors r+ rollup

[bors]

📌 Commit ecf46d1 has been approved by Mark-Simulacrum