Correct cpu_arch value for x86 arch.

GN build use x86 instead ia32 for cpu_arch, otherwise the x86 files
won't be included to build.

BUG=chromium:455167
R=andrew@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/38869004

Patch from Halton Huo <halton.huo@intel.com>.

git-svn-id: http://webrtc.googlecode.com/svn/deps/third_party/openmax@8246 4adac7df-926f-26a2-2b94-8c16560cd09d