barrier.h: add generic smp_mb implementation
This missing define causes build failures on s390:
src/include/liburing.h:298: undefined reference to `io_uring_smp_mb'
@@ -76,6 +76,7 @@ do { \
* Add arch appropriate definitions. Be safe and use full barriers for
* archs we don't have support for.
*/
+#define io_uring_smp_mb() __sync_synchronize()
#define io_uring_smp_rmb() __sync_synchronize()
#define io_uring_smp_wmb() __sync_synchronize()
#endif /* defined(__x86_64__) || defined(__i386__) */
