cmd/link/internal/ld: don't leave files open in a loop

Noticed by Ingo Oeser in his review of CL 168321.

Change-Id: I2f39db675a7c22b395062a11903657a9d0d1956d
Reviewed-on: https://go-review.googlesource.com/c/go/+/168560


TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>