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
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 ()