runtime: skip TestCgoCallbackGC on linux/mips64x

Builder is too slow. This test passed on builder machines but took
15+ min.

Change-Id: Ief9d67ea47671a57e954e402751043bc1ce09451
Reviewed-on: https://go-review.googlesource.com/22798Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/crash_cgo_test.go

@@ -51,6 +51,8 @@ func TestCgoCallbackGC(t *testing.T) {
 			t.Skip("see golang.org/issue/11990")
 		case runtime.GOOS == "linux" && runtime.GOARCH == "arm":
 			t.Skip("too slow for arm builders")
+		case runtime.GOOS == "linux" && (runtime.GOARCH == "mips64" || runtime.GOARCH == "mips64le"):
+			t.Skip("too slow for mips64x builders")
 		}
 	}
 	got := runTestProg(t, "testprogcgo", "CgoCallbackGC")