runtime: fix 386, arm build

CL 68150047 lost runtime.panicdivide.

TBR=dfc
CC=golang-codereviews
https://golang.org/cl/68610043

src/pkg/runtime/panic.c

@@ -487,3 +487,9 @@ runtime·Goexit(void)
 	rundefer();
 	runtime·goexit();
 }
+
+void
+runtime·panicdivide(void)
+{
+	runtime·panicstring("integer divide by zero");
+}