This change broke divmod.go on all arm platforms.

««« original CL description
cmd/gc: change interface representation: only pointers in data word

Note that there are various cleanups that can be made if we keep
this change, but I do not want to start making changes that
depend on this one until the 1.4 cycle closes.

Fixes #8405.

LGTM=r
R=golang-codereviews, adg, r, bradfitz
CC=golang-codereviews, iant
https://golang.org/cl/130240043
»»»

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/133810043