Fix ARM64 build.

Provide the correct defines and APIs on ARM64, which always has NEON.

TEST=local ARM64 Chromium build passes.
BUG=chromium:415393
R=rtoy@google.com

Review URL: https://webrtc-codereview.appspot.com/26739004

git-svn-id: http://webrtc.googlecode.com/svn/deps/third_party/openmax@7391 4adac7df-926f-26a2-2b94-8c16560cd09d