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 #8405.

LGTM=bradfitz, dave
R=golang-codereviews, bradfitz
CC=dave, golang-codereviews, r
https://golang.org/cl/133830043