cpu: Turn cpu_unassigned_access() into a CPUState hook

Use it for all targets, but be careful not to pass invalid CPUState.
cpu_single_env can be NULL, e.g. on Xen.

Signed-off-by: Andreas Färber <afaerber@suse.de>
diff --git a/target-alpha/mem_helper.c b/target-alpha/mem_helper.c
index 3d2cd61..7160a1c 100644
--- a/target-alpha/mem_helper.c
+++ b/target-alpha/mem_helper.c
@@ -109,11 +109,15 @@
     cpu_loop_exit(env);
 }
 
-void cpu_unassigned_access(CPUAlphaState *env, hwaddr addr,
-                           int is_write, int is_exec, int unused, int size)
+void alpha_cpu_unassigned_access(CPUState *cs, hwaddr addr,
+                                 bool is_write, bool is_exec, int unused,
+                                 unsigned size)
 {
+    AlphaCPU *cpu = ALPHA_CPU(cs);
+    CPUAlphaState *env = &cpu->env;
+
     env->trap_arg0 = addr;
-    env->trap_arg1 = is_write;
+    env->trap_arg1 = is_write ? 1 : 0;
     dynamic_excp(env, 0, EXCP_MCHK, 0);
 }