gc: put limit on size of exported recursive interface

Prevents edge-case recursive types from consuming excessive memory.

Fixes #1909.

R=golang-dev, lvd, rsc
CC=golang-dev
https://golang.org/cl/5504108

src/cmd/gc/fmt.c

@@ -1450,7 +1450,7 @@ Tconv(Fmt *fp)
 	if(t == T)
 		return fmtstrcpy(fp, "<T>");
 
-	if(t->trecur > 4)
+	if(t->trecur > 4 || fp->nfmt > 1000)
 		return fmtstrcpy(fp, "<...>");
 
 	t->trecur++;