breaks 386 build

««« original CL description
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

»»»

R=lvd, lvd
CC=golang-dev
https://golang.org/cl/5534049