cmd/link/internal/ld: deflake TestWindowsIssue36495

Over a dozen of the ld tests were missing closes. That was less
obvious before CL 299670 started using T.TempDir instead, which fails
a test when the tempdir can't be cleaned up (as it can't on Windows
when things are still open), insteading of leaving tempdirs around on
disk after the test.

Most of the missing closes were fixed in CL 299670, but the builders
helpfully pointed out that I missed at least this one.

Change-Id: I35f695bb7cbfba31e16311c5af965c148f9d7943
Reviewed-on: https://go-review.googlesource.com/c/go/+/299929
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Trust: Dan Scales <danscales@google.com>
Trust: Alex Brainman <alex.brainman@gmail.com>

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

@@ -1297,6 +1297,7 @@ func main() {
   fmt.Println("Hello World")
 }`
 	f := gobuild(t, dir, prog, NoOpt)
+	defer f.Close()
 	exe, err := pe.Open(f.path)
 	if err != nil {
 		t.Fatalf("error opening pe file: %v", err)