Config changes to support target_os="fuchsia"

build/toolchain/fuchsia/BUILD.gn

+# Copyright 2017 The Chromium 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")
+import("//build/config/fuchsia/config.gni")
+
+clang_toolchain("x64") {
+  assert(current_cpu == "x64")
+  assert(host_os == "linux")
+
+  toolchain_args = {
+    current_cpu = "x64"
+    current_os = "fuchsia"
+  }
+}