run-system returns invalid codes

```koka
import std/os/process

fun main()
  val status = run-system("exit 3")
  println(status)
```
will return... 768. "exit 1" will return 256.

FTR, 
```
$ sh -c "exit 1"
$ echo $?
1

$ sh -c "exit 3"
$ echo $?
3
```