From the description of CL 60190043

debug/macho: Add support for opening fat/universal binaries.

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

Update #7250

LGTM=iant
R=golang-codereviews, gobot, dsymonds, iant
CC=golang-codereviews
https://golang.org/cl/61720044