Flush stdout after writeln (fixes issue #13241)

Followup to pull https://github.com/D-Programming-Language/phobos/pull/2334

Fixes: https://issues.dlang.org/show_bug.cgi?id=13241

Not sure if the `.flush` call should be inside a version block for DMC, since apparently this isn't an issue on other platforms.
> Never tried building dmd under Wine. Maybe I should give that a shot one of these days.

That reminds me. AFAIK, std.datetime's unit tests won't work there, because the wine folks did screwy stuff with the time zones in their registry (the keys don't match).  I don't think that I ever added a way to get around that. I probably should one of these days (though they really should make their registry match IMHO).