sim: callback: extend syscall interface to handle 7 args

The Linux syscall interface, depending on architecture, handles up to
7 arguments.  Extend the callback API to handle those.

include/sim/callback.h

@@ -241,7 +241,7 @@ typedef struct cb_syscall {
   /* The target's value of what system call to perform.  */
   int func;
   /* The arguments to the syscall.  */
-  long arg1, arg2, arg3, arg4;
+  long arg1, arg2, arg3, arg4, arg5, arg6, arg7;
 
   /* The result.  */
   long result;