Add cmake toolchain for x86 support on linux.

- Properly respond to detection of i386 and x86 as the CPU.
- Restrict -Wshadow to C files: it silences a large number of
  warnings caused by test code.

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

Change-Id: Ia15ae8ed0fa90aec587818bda778a638c811168d

build/cmake/aom_configure.cmake

diff --git a/build/cmake/aom_configure.cmake b/build/cmake/aom_configure.cmake
index 7aae928..f0f026a 100644
--- a/build/cmake/aom_configure.cmake
+++ b/build/cmake/aom_configure.cmake
@@ -29,10 +29,15 @@
               "      CMAKE_SYSTEM_PROCESSOR=${CMAKE_SYSTEM_PROCESSOR}\n"
               "      CMAKE_GENERATOR=${CMAKE_GENERATOR}\n")
     endif ()
+  elseif ("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "i386" OR
+          "${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "x86")
+    set(AOM_TARGET_CPU "x86")
   endif ()
 endif ()
 
+message("--- aom_configure: Detected CPU: ${AOM_TARGET_CPU}")
 set(AOM_TARGET_SYSTEM ${CMAKE_SYSTEM_NAME})
+
 if (NOT EXISTS "${AOM_ROOT}/build/cmake/targets/${AOM_TARGET_CPU}.cmake")
   message(FATAL_ERROR "No RTCD template for ${AOM_TARGET_CPU}. Create one, or "
           "add -DAOM_TARGET_CPU=generic to your cmake command line for a "
@@ -68,12 +73,15 @@
   add_compiler_flag_if_supported("-Wfloat-conversion")
   add_compiler_flag_if_supported("-Wimplicit-function-declaration")
   add_compiler_flag_if_supported("-Wpointer-arith")
-  add_compiler_flag_if_supported("-Wshadow")
   add_compiler_flag_if_supported("-Wsign-compare")
   add_compiler_flag_if_supported("-Wtype-limits")
   add_compiler_flag_if_supported("-Wuninitialized")
   add_compiler_flag_if_supported("-Wunused")
   add_compiler_flag_if_supported("-Wvla")
+
+  # Add -Wshadow only for C files to avoid massive gtest warning spam.
+  add_c_flag_if_supported("-Wshadow")
+
   if (ENABLE_WERROR)
     add_compiler_flag_if_supported("-Werror")
   endif ()

build/cmake/aom_optimization.cmake

diff --git a/build/cmake/aom_optimization.cmake b/build/cmake/aom_optimization.cmake
index dc304b3..701abff 100644
--- a/build/cmake/aom_optimization.cmake
+++ b/build/cmake/aom_optimization.cmake
@@ -81,6 +81,8 @@
   elseif ("${AOM_TARGET_CPU}" STREQUAL "x86")
     if ("${AOM_TARGET_SYSTEM}" STREQUAL "Darwin")
       set(objformat "macho32")
+    elseif ("${AOM_TARGET_SYSTEM}" STREQUAL "Linux")
+      set(objformat "elf32")
     elseif ("${AOM_TARGET_SYSTEM}" STREQUAL "Windows")
       set(objformat "win32")
     else ()

build/cmake/toolchains/x86-linux.cmake

diff --git a/build/cmake/toolchains/x86-linux.cmake b/build/cmake/toolchains/x86-linux.cmake
new file mode 100644
index 0000000..077c8f3
--- /dev/null
+++ b/build/cmake/toolchains/x86-linux.cmake
@@ -0,0 +1,14 @@
+##
+## 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 "Linux")
+set(CMAKE_C_COMPILER_ARG1 "-m32")
+set(CMAKE_CXX_COMPILER_ARG1 "-m32")