Set bootstrap toolchain's toolchain_os to target_os

src/trusted/service_runtime/linux/toolchain/BUILD.gn

 # Copyright (c) 2015 The Native Client Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/toolchain/gcc_toolchain.gni")
 
 # This toolchain's sole purpose is to compile nacl_helper_bootstrap.c.
 # That requires a separate toolchain because that's the most reasonable way
 # to instantiate the standard configs in a context where use_gold=false
 # when the main build is using use_gold=true.  See ../BUILD.gn for details.
 
 template("nacl_bootstrap_toolchain") {
   clang_toolchain(target_name) {
     toolchain_cpu = invoker.toolchain_cpu
-    toolchain_os = "linux"
+    toolchain_os = target_os
     use_gold = false
   }
 }
 
 nacl_bootstrap_toolchain("nacl_bootstrap_x64") {
   toolchain_cpu = "x64"
 }
 
 nacl_bootstrap_toolchain("nacl_bootstrap_x86") {
   toolchain_cpu = "x86"
 }
 
 nacl_bootstrap_toolchain("nacl_bootstrap_arm") {
   toolchain_cpu = "arm"
 }