cmd/asm: remove object file if assembly fails.

Just an oversight. Plus the code had an unnecessary call to os.Exit
that now has a purpose.

Fixes #10372.

Change-Id: I456018f3a01ca05b4501c7f8a4961d48ab8c5e16
Reviewed-on: https://go-review.googlesource.com/8651Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/asm/main.go

@@ -57,7 +57,8 @@ func main() {
 	var ok bool
 	pList.Firstpc, ok = parser.Parse()
 	if !ok {
-		log.Fatalf("asm: assembly of %s failed", flag.Arg(0))
+		log.Printf("asm: assembly of %s failed", flag.Arg(0))
+		os.Remove(*flags.OutputFile)
 		os.Exit(1)
 	}
 	obj.Writeobjdirect(ctxt, output)