New testdata was created from existing using:
$ lipo gcc-386-darwin-exec gcc-amd64-darwin-exec -create -output fat-gcc-386-amd64-darwin-exec

Fixes #7250.

LGTM=dave
R=golang-codereviews, dave, josharian, bradfitz
CC=golang-codereviews
https://golang.org/cl/60190043