Implemented subtraction

src/build/asm/Mnemonic.go

@@ -10,6 +10,7 @@ const (
 	LABEL
 	MOVE
 	RETURN
+	SUB
 	SYSCALL
 )
 
@@ -34,6 +35,9 @@ func (m Mnemonic) String() string {
 	case RETURN:
 		return "return"
 
+	case SUB:
+		return "sub"
+
 	case SYSCALL:
 		return "syscall"
 	}