cmd/vet has a known deficiency in its handling of fmt.Formatters.
This causes a spurious printf error only for non-host platforms.
Since cmd/vet/all may get run on any given platform,
whitelists cannot help here.

Work around the issue by skipping printf tests entirely
for non-host platforms.

Work around the one known acceptable false positive from vet
by whitelisting the file that contains it.

Change-Id: Id74b3d4db0519cf9a670a065683715f856266e45
Reviewed-on: https://go-review.googlesource.com/36936Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>