Auto merge of #115719 - tgross35:atomic-from-ptr, r=dtolnay

Stabilize `atomic_from_ptr`

This stabilizes `atomic_from_ptr` and moves the const gate to `const_atomic_from_ptr`. Const stability is blocked on `const_mut_refs`.

Tracking issue:  #108652

Newly stable API:

```rust
// core::atomic

impl AtomicBool { pub unsafe fn from_ptr<'a>(ptr: *mut bool) -> &'a AtomicBool; }

impl<T> AtomicPtr<T> { pub unsafe fn from_ptr<'a>(ptr: *mut *mut T) -> &'a AtomicPtr<T>; }

impl AtomicU8    { pub unsafe fn from_ptr<'a>(ptr: *mut u8)    -> &'a AtomicU8;    }
impl AtomicU16   { pub unsafe fn from_ptr<'a>(ptr: *mut u16)   -> &'a AtomicU16;   }
impl AtomicU32   { pub unsafe fn from_ptr<'a>(ptr: *mut u32)   -> &'a AtomicU32;   }
impl AtomicU64   { pub unsafe fn from_ptr<'a>(ptr: *mut u64)   -> &'a AtomicU64;   }
impl AtomicUsize { pub unsafe fn from_ptr<'a>(ptr: *mut usize) -> &'a AtomicUsize; }

impl AtomicI8    { pub unsafe fn from_ptr<'a>(ptr: *mut i8)    -> &'a AtomicI8;    }
impl AtomicI16   { pub unsafe fn from_ptr<'a>(ptr: *mut i16)   -> &'a AtomicI16;   }
impl AtomicI32   { pub unsafe fn from_ptr<'a>(ptr: *mut i32)   -> &'a AtomicI32;   }
impl AtomicI64   { pub unsafe fn from_ptr<'a>(ptr: *mut i64)   -> &'a AtomicI64;   }
impl AtomicIsize { pub unsafe fn from_ptr<'a>(ptr: *mut isize) -> &'a AtomicIsize; }
```

Author: bors