Stabilize `strict_provenance_atomic_ptr` feature This closes [tracking issue](https://github.com/rust-lang/rust/issues/99108) and stabilises `AtomicPtr::{fetch_ptr_add, fetch_ptr_sub, fetch_byte_add, fetch_byte_sub, fetch_or, fetch_and, fetch_xor}` --- EDIT: FCP completed at https://github.com/rust-lang/rust/issues/99108#issuecomment-3168260347