Fixes #130474

This is my first contribution, so I'll try to be as descriptive as possible of my process and thoughts.

• Environment: Cross compiled in Macbook M1 Air
• Testing: I replicated the bug and checked fixes with ./x doc library/core --target x86_64-unknown-linux-gnu (building the std doc didn't seem to apply my local changes so I used the core doc) Check the screenshots below.
• Diffs: I added #[cfg(any(...,doc))] to all required structs and implementations so the types would compile. Applyingtarget_has_atomic_equal_alignment part wasn't explicitly necessary, but adding it reveals the whole methods (specifically from_mut and from_mut_slice).
• Thoughts: Also wondered if additional messages should be written to notify the available targets, but the documentation already included the following sentence that seemed enough.
  • Note: This type is only available on platforms that support atomic loads and stores of u128/i128

[Before]

[After]

After the first work, doc tests were failing in CI, so I added #[$cfg_cas] to all methods in the atomic_int macro to cover them. But I'm not really sure if all these changes are necessary (and appropriate).
This also has a side-effect of stating "Available on target_has_atomic=128 only." for all methods with the attribute. Also not sure if this is a good behavior..