Though I am not aware of any architecture where this matters, it
is technically incorrect to call these stdio functions as if they
had no result.
The QBE documentation says
> Unless the called function does not return a value, a return
> temporary must be specified, even if it is never used afterwards.
so we should follow it in the tests as well.
The vararg tests had to be changed because
va_list is 32-bit wide on arm. The astute
reader will notice that the way we pass
va_list values is wrong, we should be using
the ':valist' type as defined below instead
of 'l'. But eh, that works for now, because
of the ABI.
type :valist = align 8 { 32 }
