For building v8 using gn on aix_ppc64, linux_s390x and linux_ppc64.

build/config/host_byteorder.gni

+# Copyright (c) 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.
+
+# This header file defines the "host_byteorder" variable.
+# Not that this is currently used only for building v8.
+# The chromium code generally assumes little-endianness.
+declare_args() {
+  host_byteorder = "undefined"
+}
+
+# Detect host byteorder
+# ppc64 can be either BE or LE
+if (host_cpu == "ppc64") {
+  if (current_os == "aix") {
+    host_byteorder = "big"
+  } else {
+    # Only use the script when absolutely necessary
+    host_byteorder =
+        exec_script("//build/config/get_host_byteorder.py", [], "trim string")
+  }
+} else if (host_cpu == "ppc" || host_cpu == "s390" || host_cpu == "s390x" ||
+           host_cpu == "mips" || host_cpu == "mips64") {
+  host_byteorder = "big"
+} else {
+  host_byteorder = "little"
+}