SI-8167 readLine shold flush output before reading input

(patch by @retronym, I just rebased it)

As reported on scala-user:

> Using `scala.Predef.readLine(text: String, args: Any*)`
> on Windows causes strange behavior. If I am leaving some
> part of `text` unforced to be flushed (say, "---\nEnter new
>  expression >") then "Enter new expression >" flushed only
> after Enter press. Still, it works fine on Ubuntu and (seems like)
> on MacOS.
>
> My workaround is to force `java.lang.System.out` (that readLine
> depends on to write welcome string) to flush output like
> "---\nEnter new expression >\n".