cmd/link: fix testdata generation

Fixes issue 8494.

LGTM=rsc
R=golang-codereviews, gobot, rsc, evankroske
CC=golang-codereviews
https://codereview.appspot.com/123040043

Committer: Russ Cox <rsc@golang.org>
cmd/gc: re-enable IfacePointerOnly

CL 130240043 did this but broke ARM, because
it made newErrorCString start allocating, so we rolled
it back in CL 133810043.

CL 133820043 removed that allocation.

Try again.

Fixes issue 8405.

LGTM=bradfitz, dave
R=golang-codereviews, bradfitz
CC=dave, golang-codereviews, r
https://codereview.appspot.com/133830043