Do not use openmax_dl for MIPS64 platform.

This fix is intended for MIPS64 Chromium Android builds, which has no openmax_dl
support at this moment.

R=andrew@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/31339004

Patch from Ljubomir Papuga <lpapuga@mips.com>.

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7983 4adac7df-926f-26a2-2b94-8c16560cd09d