runtime: fix sigaltstack test Skipf format string

Change-Id: I9485ecbd9d546893e4f0db846b08d835fa7515d7
Reviewed-on: https://go-review.googlesource.com/18140Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/cgo/test/sigaltstack.go

@@ -56,7 +56,7 @@ import (
 func testSigaltstack(t *testing.T) {
 	switch {
 	case runtime.GOOS == "solaris", runtime.GOOS == "darwin" && (runtime.GOARCH == "arm" || runtime.GOARCH == "arm64"):
-		t.Skipf("switching signal stack not implemented on %s/s", runtime.GOOS, runtime.GOARCH)
+		t.Skipf("switching signal stack not implemented on %s/%s", runtime.GOOS, runtime.GOARCH)
 	}
 
 	C.changeSignalStack()