cmd/link: close input files when copying to temporary directory

Fixes #29110

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>

src/cmd/link/internal/ld/lib.go

@@ -1017,6 +1017,7 @@ func hostobjCopy() (paths []string) {
 			if err != nil {
 				Exitf("cannot reopen %s: %v", h.pn, err)
 			}
+			defer f.Close()
 			if _, err := f.Seek(h.off, 0); err != nil {
 				Exitf("cannot seek %s: %v", h.pn, err)
 			}