Stabilize const_atomic_from_ptr

The API is already stable since [1], but const stability was blocked on
`const_mut_refs`. Since that was recently stabilized, const stabilize
the following:

    // core::atomic

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

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

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

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

Closes: <#108652>
[1]: <#115719>

library/core/src/sync/atomic.rs

@@ -469,7 +469,8 @@ impl AtomicBool {
     /// [valid]: crate::ptr#safety
     /// [Memory model for atomic accesses]: self#memory-model-for-atomic-accesses
     #[stable(feature = "atomic_from_ptr", since = "1.75.0")]
-    #[rustc_const_unstable(feature = "const_atomic_from_ptr", issue = "108652")]
+    #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
+    #[rustc_const_stable(feature = "const_atomic_from_ptr", since = "CURRENT_RUSTC_VERSION")]
     pub const unsafe fn from_ptr<'a>(ptr: *mut bool) -> &'a AtomicBool {
         // SAFETY: guaranteed by the caller
         unsafe { &*ptr.cast() }
@@ -1264,7 +1265,8 @@ impl<T> AtomicPtr<T> {
     /// [valid]: crate::ptr#safety
     /// [Memory model for atomic accesses]: self#memory-model-for-atomic-accesses
     #[stable(feature = "atomic_from_ptr", since = "1.75.0")]
-    #[rustc_const_unstable(feature = "const_atomic_from_ptr", issue = "108652")]
+    #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
+    #[rustc_const_stable(feature = "const_atomic_from_ptr", since = "CURRENT_RUSTC_VERSION")]
     pub const unsafe fn from_ptr<'a>(ptr: *mut *mut T) -> &'a AtomicPtr<T> {
         // SAFETY: guaranteed by the caller
         unsafe { &*ptr.cast() }
@@ -2262,7 +2264,8 @@ macro_rules! atomic_int {
             /// [valid]: crate::ptr#safety
             /// [Memory model for atomic accesses]: self#memory-model-for-atomic-accesses
             #[stable(feature = "atomic_from_ptr", since = "1.75.0")]
-            #[rustc_const_unstable(feature = "const_atomic_from_ptr", issue = "108652")]
+            #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
+            #[rustc_const_stable(feature = "const_atomic_from_ptr", since = "CURRENT_RUSTC_VERSION")]
             pub const unsafe fn from_ptr<'a>(ptr: *mut $int_type) -> &'a $atomic_type {
                 // SAFETY: guaranteed by the caller
                 unsafe { &*ptr.cast() }