arm64/arm64asm: fix vet warning to fix build on Go 1.10

Change-Id: Iba218f4da109845f3b5abb66100b39fdc03582f5
Reviewed-on: https://go-review.googlesource.com/84215
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

arm64/arm64asm/objdumpext_test.go

@@ -35,7 +35,7 @@ func testObjdumpArch(t *testing.T, generate func(func([]byte)), arch Mode) {
 func checkObjdumpAarch64(t *testing.T) {
 	out, err := exec.Command(objdumpPath, "-i").Output()
 	if err != nil {
-		t.Skip("cannot run objdump: %v\n%s", err, out)
+		t.Skipf("cannot run objdump: %v\n%s", err, out)
 	}
 	if !strings.Contains(string(out), "aarch64") {
 		t.Skip("objdump does not have aarch64 support")