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
3 files changed