Update the original change but do not read interface types in
the arguments area.  Once the arguments area is zeroed as the
locals area is we can safely read interface type values there
too.

««« original CL description
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://golang.org/cl/12785045
»»»

R=khr, golang-dev, khr
CC=golang-dev
https://golang.org/cl/13010045
»»»

R=khr, khr
CC=golang-dev
https://golang.org/cl/13073045