Fix build under macOS arm64

Closes #685

macosx/JackAtomic_os.h

@@ -48,9 +48,7 @@ static inline int CAS(register UInt32 value, register UInt32 newvalue, register
     return result;
 }
 
-#endif
-
-#if defined(__i386__) || defined(__x86_64__)
+#elif defined(__i386__) || defined(__x86_64__)
 
 #define LOCK "lock ; "
 
@@ -67,6 +65,13 @@ static inline char CAS(volatile UInt32 value, UInt32 newvalue, volatile void* ad
     return ret;
 }
 
+#else
+
+static inline char CAS(volatile UInt32 value, UInt32 newvalue, volatile void* addr)
+{
+    return __sync_bool_compare_and_swap ((UInt32*)addr, value, newvalue);
+}
+
 #endif
 
 #endif