Current versions of gccgo issue a duplicate definition error when both
a definition and an empty declaration occur. Use build tags to avoid
that case for the issue9400 subdirectory.

Change-Id: I18517af87bab05e9ca43f2f295459cf34347c317
Reviewed-on: https://go-review.googlesource.com/119896
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>