For some target languages, gobind will need to output multiple files.
(e.g. objective-c) -output flag is not compatible with such cases.
Change-Id: I6da6109ae75c3aaa7d7379d411df2bb14c88bbc2
Reviewed-on: https://go-review.googlesource.com/4120
Reviewed-by: David Crawshaw <crawshaw@golang.org>