undo CL 12785045 / 71ce80dc4195

This has broken the 32-bit builds.

??? original CL description
cmd/gc, runtime: use type information to scan interface values

R=golang-dev, rsc, dvyukov
CC=golang-dev
https://codereview.appspot.com/12785045
???

R=khr, golang-dev, khr
CC=golang-dev
https://codereview.appspot.com/13010045