Auto merge of #84052 - RalfJung:libcore-miri, r=Mark-Simulacrum

fix Miri errors in libcore doctests

Now that Miri can run doctests, it found some issues in the libcore doctests:
* The `AtomicPtr` tests accessed dangling memory! `AtomicPtr::new(&mut 10);` makes the `10` a temporary that is deallocated after the end of this expression.
* The tests for `set_ptr_value` used `&array[0] as *const _` to get a pointer to the array; this needs to be `array.as_ptr()` instead (Cc https://github.com/rust-lang/unsafe-code-guidelines/issues/134).
* I reduced a buffer size in a `MaybeUninit` test to make it less slow in Miri, and added a spin loop hint to fix a diverging loop in Miri.

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