cmd/asm: close file before remove

Windows doesn't remove an open file.

Fixes #17345

Change-Id: I283930c7d6eb3bc09ad208191afefe989804ce32
Reviewed-on: https://go-review.googlesource.com/30430
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/asm/main.go

@@ -80,6 +80,7 @@ func main() {
 		} else {
 			log.Print("assembly failed")
 		}
+		out.Close()
 		os.Remove(*flags.OutputFile)
 		os.Exit(1)
 	}