spec: Go has no 'reference types'

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8288044
cmd/gc: be more tolerant with recursive types when checking map types.

A nested TFORW type would push algtype1 into an impossible case.

Fixes issue 5125.

R=golang-dev, daniel.morsing
CC=golang-dev
https://codereview.appspot.com/8213043