runtime: fix mkasmh.sh for arm?

The builder is Debian, so maybe running dash,
the shell that time forgot.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5600053

src/pkg/runtime/mkasmh.sh

@@ -3,7 +3,7 @@
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
 
-trap "rm -f arch_GOARCH.h defs_GOOS_GOARCH.h os_GOOS.h signals_GOOS.h" EXIT SIGINT SIGTERM
+trap "rm -f arch_GOARCH.h defs_GOOS_GOARCH.h os_GOOS.h signals_GOOS.h" EXIT INT TERM
 set -e
 
 SYS=$1