clang on linux prefers no args for rep movsl/stosl. ifdef for C that removes namespace.  extern c allows c++ to be used to compile, including with namespace, but C program can link libyuv
BUG=none
TEST=none
Review URL: http://webrtc-codereview.appspot.com/320004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@101 16f28f9a-4ce2-e073-06de-1de4eb20be90