add nacl_toolchain to //build and flip to using it in the GN build.

build/toolchain/nacl_toolchain.gni

+# 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/toolchain/gcc_toolchain.gni")
+
+# This template defines a NaCl toolchain.
+#
+# It requires the following variables specifying the executables to run:
+#  - cc
+#  - cxx
+#  - ar
+#  - ld
+# and the following which is used in the toolchain_args
+#  - toolchain_cpu  (What "current_cpu" should be set to when invoking a
+#                    build using this toolchain.)
+
+template("nacl_toolchain") {
+  assert(defined(invoker.cc), "nacl_toolchain() must specify a \"cc\" value")
+  assert(defined(invoker.cxx), "nacl_toolchain() must specify a \"cxx\" value")
+  assert(defined(invoker.ar), "nacl_toolchain() must specify a \"ar\" value")
+  assert(defined(invoker.ld), "nacl_toolchain() must specify a \"ld\" value")
+  assert(defined(invoker.toolchain_cpu),
+         "nacl_toolchain() must specify a \"toolchain_cpu\"")
+
+  toolchain_os = "nacl"
+  if (defined(invoker.is_clang)) {
+    is_clang = invoker.is_clang
+  }
+  if (defined(invoker.executable_extension)) {
+    executable_extension = invoker.executable_extension
+  } else {
+    executable_extension = ".nexe"
+  }
+  toolchain_cpu = invoker.toolchain_cpu
+
+  cc = invoker.cc
+  cxx = invoker.cxx
+  ar = invoker.ar
+  ld = invoker.ld
+  if (defined(invoker.postlink)) {
+    postlink = invoker.postlink
+  }
+  if (defined(invoker.link_outputs)) {
+    link_outputs = invoker.link_outputs
+  }
+
+  # We do not wish to suport shared builds with the NaCl toolchains.
+  is_component_build = false
+
+  gcc_toolchain(target_name) {
+    rebuild_define = "NACL_TC_REV=" + invoker.toolchain_revision
+    if (defined(invoker.deps)) {
+      deps = invoker.deps
+    }
+  }
+}