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.
This is wrong. Flushing should only happen if the stream is linebuffered (otherwise block writes will be veeeeery  sloooowwwww). For reference, here's the DMC implementation of vprintf:
```
int vprintf(const char *format,va_list args)
{   int status;

    __fp_lock(stdout);
    status = _pformat((fp_t)_fputc_nlock,stdout,format,args);
    __fp_unlock(stdout);
    if (stdout->_flag & _IOLBF)
            fflush(stdout);
    return status;
}
```