This commit fixes error messages displayed on aix/ppc64 with external
linking.

Change-Id: I5311d36f30394be717827891e070db249482814a
Reviewed-on: https://go-review.googlesource.com/c/151041Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>