runtime: fix darwin build

Add place holder for runtime.osyield.

R=golang-dev
CC=golang-dev
https://golang.org/cl/5149049

src/pkg/runtime/darwin/thread.c

@@ -496,3 +496,9 @@ runtime·sigpanic(void)
 	}
 	runtime·panicstring(runtime·sigtab[g->sig].name);
 }
+
+// TODO(rsc): place holder to fix build.
+void
+runtime·osyield(void)
+{
+}