Turns out it *is* needed because the cmd/link tests expect to find their own files.

««« original CL description
misc/nacl: exclude cmd/link from the test zip.

It does not appear to be necessary, and cmd/link does not appear in release branches.

LGTM=rsc
R=adg, rsc
CC=golang-codereviews
https://golang.org/cl/176900044
»»»

TBR=rsc
R=adg, rsc
CC=golang-codereviews
https://golang.org/cl/175870045