runtime/_mkmalloc: fix log.Fatal formatting directive

Change-Id: I9b9b9dbde440c3a24599efd55ef6f85a6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/724281
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>

Author: FiloSottile

src/runtime/_mkmalloc/mkmalloc.go

@@ -288,7 +288,7 @@ func inline(config generatorConfig) []byte {
 			case foldCondition:
 				stamped = foldIfCondition(stamped, repl.from, repl.to)
 			default:
-				log.Fatal("unknown op kind %v", repl.kind)
+				log.Fatalf("unknown op kind %v", repl.kind)
 			}
 		}