Split the GN 'snapshot_toolchain' logic out into an include file.

snapshot_toolchain.gni

+# Copyright 2014 The Chromium Authors. All rights reserved.

[jochen (gone - plz use gerrit), 2015/09/21 15:10:49]

please use the v8 license header. Essentially "The V8 project authors" instead
of "THe Chromium Authors"

+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# The snapshot needs to be compiled for the host, but compiled with
+# a toolchain that matches the bit-width of the target.
+
+# TODO(GYP): For now we only support 32-bit little-endian target builds from an
+# x64 Linux host. Eventually we need to support all of the host/target
+# configurations v8 runs on.
+if (host_cpu == "x64" && host_os == "linux") {
+  if (target_cpu == "arm" || target_cpu == "mipsel" || target_cpu == "x86") {
+    snapshot_toolchain = "//build/toolchain/linux:clang_x86"
+  } else if (target_cpu == "x64") {
+    snapshot_toolchain = "//build/toolchain/linux:clang_x64"
+  } else {
+    assert(false, "Need environment for this arch")
+  }
+} else {
+  snapshot_toolchain = default_toolchain
+}
+
+