Start to fusl toolchain

build/toolchain/fusl/BUILD.gn

+# Copyright 2016 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")
+
+gcc_toolchain("fusl_x64") {
+  prefix = rebase_path("//third_party/llvm-build/Release+Asserts/bin",
+                       root_build_dir)
+
+  cc = "$prefix/clang"
+  cxx = "$prefix/clang++"
+
+  readelf = "readelf"
+  nm = "nm"
+  ar = "ar"
+  ld = cc
+
+  toolchain_cpu = "x64"
+  toolchain_os = "linux"
+  is_clang = true
+
+  use_goma = false
+  use_ccache = false
+}