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 #7111.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/65720045

src/cmd/link/prog_test.go

@@ -112,7 +112,7 @@ func cloneProg(p *Prog) *Prog {
 	for i, seg := range p.Segments {
 		q.Segments[i] = cloneSegment(seg)
 	}
-	return p
+	return q
 }
 
 // cloneSegment returns a deep copy of seg.