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>

configure

@@ -5894,6 +5894,7 @@ case "$target_name" in
     TARGET_BASE_ARCH=i386
   ;;
   alpha)
+    mttcg="yes"
   ;;
   arm|armeb)
     TARGET_ARCH=arm

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"