cmd/compile: update fmt_test (fix build for long-running tests)

Follow-up on https://golang.org/cl/136397.

Change-Id: Ib0df690847c7c92d8de406dadc16a10507bfda39
Reviewed-on: https://go-review.googlesource.com/c/139059Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/fmt_test.go

@@ -700,6 +700,7 @@ var knownFormats = map[string]string{
 	"int8 %x":                                         "",
 	"interface{} %#v":                                 "",
 	"interface{} %T":                                  "",
+	"interface{} %p":                                  "",
 	"interface{} %q":                                  "",
 	"interface{} %s":                                  "",
 	"interface{} %v":                                  "",