fix Miri errors in libcore doctests

[RalfJung]

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 Stacked Borrows: raw pointer usable only for T too strict? unsafe-code-guidelines#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.

[rust-highfive]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

@bors r+ rollup

[bors]

📌 Commit b35ac69 has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit b35ac69 with merge 72c63de...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 72c63de to master...