cmd/asm: use strings.TrimSuffix

Change-Id: If08ce3f8534f5fb7ce97ec124bfeff937e65f63b
Reviewed-on: https://go-review.googlesource.com/c/go/+/435943
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Run-TryBot: Cherry Mui <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: cuiweixie

src/cmd/asm/internal/flags/flags.go

@@ -84,9 +84,7 @@ func Parse() {
 			flag.Usage()
 		}
 		input := filepath.Base(flag.Arg(0))
-		if strings.HasSuffix(input, ".s") {
-			input = input[:len(input)-2]
-		}
+		input = strings.TrimSuffix(input, ".s")
 		*OutputFile = fmt.Sprintf("%s.o", input)
 	}
 }