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

test/arm/test_call.rb

@@ -19,8 +19,9 @@ module Arm
       Risc.machine.boot
       bin = Parfait::BinaryCode.new(1)
       Risc::Position.set(bin , 0x20)
+      Risc::Position.set(@binary , 0)
       code = @machine.call(	bin ,{} )#this jumps to the next instruction
-      Risc::Position.set(code , 0, 1)
+      Risc::Position.set(code , 0, @binary)
       assert_code code , :call, [0x09,0x0,0x0,0xeb]
     end
     def test_swi