[sanitizer] Use __atomic_load/store() built-ins for generic 32-bit targets

Simplifies the code and fixes the build on SPARC.
See discussion in: http://lists.llvm.org/pipermail/llvm-dev/2020-October/145937.html

Author: glaubitz (John Paul Adrian Glaubitz)
Reviewed-in: https://reviews.llvm.org/D89940

Author: dvyukov

compiler-rt/lib/sanitizer_common/sanitizer_atomic_clang_other.h

@@ -50,11 +50,8 @@ inline typename T::Type atomic_load(
       __sync_synchronize();
     }
   } else {
-    // 64-bit load on 32-bit platform.
-    // Gross, but simple and reliable.
-    // Assume that it is not in read-only memory.
-    v = __sync_fetch_and_add(
-        const_cast<typename T::Type volatile *>(&a->val_dont_use), 0);
+    __atomic_load(const_cast<typename T::Type volatile *>(&a->val_dont_use), &v,
+                  __ATOMIC_SEQ_CST);
   }
   return v;
 }
@@ -79,16 +76,7 @@ inline void atomic_store(volatile T *a, typename T::Type v, memory_order mo) {
       __sync_synchronize();
     }
   } else {
-    // 64-bit store on 32-bit platform.
-    // Gross, but simple and reliable.
-    typename T::Type cmp = a->val_dont_use;
-    typename T::Type cur;
-    for (;;) {
-      cur = __sync_val_compare_and_swap(&a->val_dont_use, cmp, v);
-      if (cur == cmp || cur == v)
-        break;
-      cmp = cur;
-    }
+    __atomic_store(&a->val_dont_use, &v, __ATOMIC_SEQ_CST);
   }
 }