Add toolchain file for x86 macos cmake builds.

BUG=https://bugs.chromium.org/p/aomedia/issues/detail?id=76

Change-Id: I14dd6704450a06898ca1bc5c0f4f9b21316c4df1

build/cmake/toolchains/x86-macos.cmake

+##
+## Copyright (c) 2017, Alliance for Open Media. All rights reserved
+##
+## This source code is subject to the terms of the BSD 2 Clause License and
+## the Alliance for Open Media Patent License 1.0. If the BSD 2 Clause License
+## was not distributed with this source code in the LICENSE file, you can
+## obtain it at www.aomedia.org/license/software. If the Alliance for Open
+## Media Patent License 1.0 was not distributed with this source code in the
+## PATENTS file, you can obtain it at www.aomedia.org/license/patent.
+##
+set(CMAKE_SYSTEM_PROCESSOR "x86")
+set(CMAKE_SYSTEM_NAME "Darwin")
+set(CMAKE_OSX_ARCHITECTURES "i386")
+set(CMAKE_C_COMPILER_ARG1 "-arch i386")
+set(CMAKE_CXX_COMPILER_ARG1 "-arch i386")
+
+# Apple tools always complain in 32 bit mode without PIC.
+set(CONFIG_PIC 1 CACHE STRING "")