Breaks the build.  Old bucket arrays kept by iterators
still need to be scanned.

««« original CL description
runtime: tell GC not to scan internal hashmap structures.
We'll do it ourselves via hash_gciter, thanks.
Fixes bug 6119.

R=golang-dev, dvyukov, cookieo9, rsc
CC=golang-dev
https://golang.org/cl/12840043
»»»

R=golang-dev
CC=golang-dev
https://golang.org/cl/12884043