Mark <*const _>::align_offset and <*mut _>::align_offset as const fn

[WaffleLapkin]

This PR marks the following APIs as const:

impl<T> *const T {
    pub const fn align_offset(self, align: usize) -> usize;
}

impl<T> *mut T {
    pub const fn align_offset(self, align: usize) -> usize;
}

const implementation simply returns usize::MAX.

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Previous discussion: #90607 (comment)

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r? @oli-obk

[oli-obk]

Thanks for doing this one first!

Please add a tracking issue that raises the concern that at compile time we will always get usize::MAX and that it is important that this gets talked about before stabilisation

[WaffleLapkin]

@oli-obk done

[oli-obk]

@bors r+ rollup

[bors]

📌 Commit f926c0e has been approved by oli-obk

[RalfJung]

Please make sure to Cc @rust-lang/wg-const-eval for every new use of const_eval_select... this one actually breaks its safety requirements.