Add target_cpu builtin variable to gclient

This CL adds target_cpu and target_cpu_only variables.  The logic is:

* If target_cpu is defined in .gclient, use it.  Otherwise, initialize it as an
  empty list.
* If not target_cpu_only, add the host arch to the list too.

detect_host_arch.py is copied directly from the Chromium repo.

BUG=807986
R=dpranke

Change-Id: I27621cbc81ad6a844648525863b92ffdd3b1d03a
Reviewed-on: https://chromium-review.googlesource.com/902441
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>

detect_host_arch.py

@@ -0,0 +1,51 @@
+#!/usr/bin/env python
+# Copyright 2014 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.
+
+"""Outputs host CPU architecture in format recognized by gyp."""
+
+import platform
+import re
+import sys
+
+
+def HostArch():
+  """Returns the host architecture with a predictable string."""
+  host_arch = platform.machine()
+
+  # Convert machine type to format recognized by gyp.
+  if re.match(r'i.86', host_arch) or host_arch == 'i86pc':
+    host_arch = 'x86'
+  elif host_arch in ['x86_64', 'amd64']:
+    host_arch = 'x64'
+  elif host_arch.startswith('arm'):
+    host_arch = 'arm'
+  elif host_arch.startswith('aarch64'):
+    host_arch = 'arm64'
+  elif host_arch.startswith('mips'):
+    host_arch = 'mips'
+  elif host_arch.startswith('ppc'):
+    host_arch = 'ppc'
+  elif host_arch.startswith('s390'):
+    host_arch = 's390'
+
+
+  # platform.machine is based on running kernel. It's possible to use 64-bit
+  # kernel with 32-bit userland, e.g. to give linker slightly more memory.
+  # Distinguish between different userland bitness by querying
+  # the python binary.
+  if host_arch == 'x64' and platform.architecture()[0] == '32bit':
+    host_arch = 'x86'
+  if host_arch == 'arm64' and platform.architecture()[0] == '32bit':
+    host_arch = 'arm'
+
+  return host_arch
+
+def DoMain(_):
+  """Hook to be called from gyp without starting a separate python
+  interpreter."""
+  return HostArch()
+
+if __name__ == '__main__':
+  print DoMain([])