`writeln` is not thread-safe about sdk

[lrhn]

Must it be?

I'd personally accept any interleaving of the characters emitted by the isolates, as long as the ones from a single isolate are in the correct order.
The Posix standard does require individual calls to be flock-ing on the FILE*, but we haven't promised that writeln is atomic, or that it's one Posix call.

If it's easy and cheap to fix, we should probably do it, just for the convenience.
If not, maybe it just needs a disclaimer.

[ntkme]

We already made print behave this way. I guess it shouldn't be too hard to make writeln for at least stdout and stderr behave the same way.