Use host instead of current OS for NaCl toolchain location

build/toolchain/nacl/BUILD.gn

 # Copyright (c) 2014 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/config/sysroot.gni")
 import("//build/toolchain/nacl_toolchain.gni")
 
 nacl_toolchain_dir = rebase_path("//native_client/toolchain", root_build_dir)
-os_toolchain_dir = "${nacl_toolchain_dir}/${current_os}_x86"
+os_toolchain_dir = "${nacl_toolchain_dir}/${host_os}_x86"
 
 # Add the toolchain revision as a preprocessor define so that sources are
 # rebuilt when a toolchain is updated.
 # Idea we could use the toolchain deps feature, but currently that feature is
 # bugged and does not trigger a rebuild.
 # https://code.google.com/p/chromium/issues/detail?id=431880
 # Calls to get the toolchain revision are relatively slow, so do them all in a
 # single batch to amortize python startup, etc.
 revisions = exec_script("//native_client/build/get_toolchain_revision.py",
                         [
@@ skipping 120 matching lines @@
   toolchain_tuple = "i686-nacl"
 }
 
 nacl_clang_toolchains("x64") {
   toolchain_tuple = "x86_64-nacl"
 }
 
 nacl_clang_toolchains("arm") {
   toolchain_tuple = "arm-nacl"
 }