Fix arm offset calculation (#152)

* Fix offset calculation

.subtracted is unreliable, and .disp has the sign anyway

* Use .subtracted for reg offsets

manticore/core/cpu/arm.py

@@ -133,10 +133,10 @@ class Armv7Operand(Operand):
             if self.is_shifted():
                 shift = self.op.shift
                 idx, carry = self.cpu._Shift(idx, shift.type, shift.value,  carry)
-            off = idx
+            off = -idx if self.op.subtracted else idx
         else:
             off = self.mem.disp
-        return -off if self.op.subtracted else off
+        return off
 
     def get_mem_base_addr(self):
         assert self.type == 'memory'