On Windows, "export" doesn't mean anything, but Windows users are the
most likely to see this message.

Fixes #15977

Change-Id: Ib09ca08a7580713cacb65d0cdc43730765c377a8
Reviewed-on: https://go-review.googlesource.com/23840Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>