We were generating format strings containing
a lone %. Vet legitimately complains:

cmd/compile/internal/gc/constFold_test.go:339: unrecognized printf verb ' '

The fix doesn't make for very readable code,
but it is simple and obviously correct.

Updates #11041

Change-Id: I90bd2d1d140887f5229752a279f7e46921472fbb
Reviewed-on: https://go-review.googlesource.com/27115
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>