Stdout should be flushed on exit

[solson]

See http://is.gd/SH4nll

This fails to print anything since "hey" in only written to the buffer and then never printed. Other BufWriters flush on drop but this isn't happening for Stdout at program exit.

[shepmaster]

Looks like a duplicate of #23191, which is closed in favor of #20012

[alexcrichton]

Indeed, thanks @shepmaster!