fix instruction positioning

use at as at and only compute difference when needed (to determine if
there is a jump in binary)
Easier to understand that way

lib/risc/position.rb

@@ -65,7 +65,7 @@ module Risc
       when Arm::Instruction , Risc::Label
         InstructionPosition.new(object,at , extra)
       else
-        ObjectPosition.new(at)
+        ObjectPosition.new(at,object)
       end
     end
   end