[release-branch.go1.11] cmd/link: close input files when copying to temporary directory

Updates #29110
Fixes #29112

Change-Id: I077d1a9caa7f4545de1418cec718c4a37ac36ef8
Reviewed-on: https://go-review.googlesource.com/c/152757


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit 35435b27)
Reviewed-on: https://go-review.googlesource.com/c/152760