Temporarily disable cpu detection in libvpx

CPU_DETECTION doesn't work in current build yet, it caused building errors
with neon build, disable it for now. Will discuss the future plan with the
team.

TBR=stefan
BUG=
TEST=
Review URL: https://webrtc-codereview.appspot.com/570006

git-svn-id: http://webrtc.googlecode.com/svn/trunk@2218 4adac7df-926f-26a2-2b94-8c16560cd09d
2 files changed