Added define operator

src/build/asm/Instruction.go

@@ -5,15 +5,5 @@ import "fmt"
 // Instruction represents a single instruction which can be converted to machine code.
 type Instruction struct {
 	Mnemonic Mnemonic
-	Data     interface{}
-}
-
-// String returns a human readable version.
-func (x *Instruction) String() string {
-	switch data := x.Data.(type) {
-	case RegisterNumber:
-		return fmt.Sprintf("%s %s, %x", x.Mnemonic, data.Register, data.Number)
-	default:
-		return x.Mnemonic.String()
-	}
+	Data     fmt.Stringer
 }