Rollup merge of rust-lang#42098 - japaric:gh42097, r=Mark-Simulacrum

core: fix unused_macro warning

when compiling the crate for a target with max-atomic-width = 0

fixes rust-lang#42097

Author: Mark-Simulacrum

src/libcore/sync/atomic.rs

@@ -918,6 +918,7 @@ impl<T> AtomicPtr<T> {
     }
 }
 
+#[cfg(target_has_atomic = "ptr")]
 macro_rules! atomic_int {
     ($stable:meta,
      $stable_cxchg:meta,