Tracking Issue for `cfg_target_has_atomic_equal_alignment`

[Amanieu]

This is a tracking issue for the target_has_atomic_equal_alignment CFG flag which was added in #76965 for Atomic*::from_mut.

It indicates whether an AtomicX has the same alignment as the equivalent X integer type. On some 32-bit platforms (e.g. x86) AtomicI64 has an alignment of 8 while i64 is only aligned to 4 bytes.

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Potential design concerns:

• Open question of whether a dedicated cfg is necessary over cfg(accessible) on the from_mut method specifically, which is what is currently gated on this cfg