target/alpha: Enable MTTCG by default

Alpha has a weak memory ordering and issues all of the required barriers.

Signed-off-by: Richard Henderson <rth@twiddle.net>
diff --git a/target/alpha/cpu.h b/target/alpha/cpu.h
index b08d160..691ac00 100644
--- a/target/alpha/cpu.h
+++ b/target/alpha/cpu.h
@@ -28,6 +28,9 @@
 
 #define CPUArchState struct CPUAlphaState
 
+/* Alpha processors have a weak memory model */
+#define TCG_GUEST_DEFAULT_MO      (0)
+
 #include "exec/cpu-defs.h"
 
 #include "fpu/softfloat.h"