cmd/link: change cloneProg to return the cloned value
cmd/link: change cloneProg to return the cloned value

The code was returning the original value rather than the cloned value
resulting in the tests not being repeatable.

Fixes issue 7111.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/65720045

Committer: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang