Add a snapshot of flutter/engine/src/build to our sdk

build/toolchain/fnl/BUILD.gn

+# Copyright 2015 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")
+
+declare_args() {
+  toolchain_prefix = ""
+}
+
+gcc_toolchain("target") {
+  assert(toolchain_prefix != "", "Must provide toolchain_prefix")
+
+  cc = "${toolchain_prefix}gcc"
+  cxx = "${toolchain_prefix}g++"
+  ar = "${toolchain_prefix}ar"
+  ld = cxx
+  readelf = "${toolchain_prefix}readelf"
+  nm = "${toolchain_prefix}nm"
+
+  toolchain_cpu = "${target_cpu}"
+  toolchain_os = "linux"
+  is_clang = is_clang
+}