Add dependencies and move the neon target to only be
specified for ARM builds so a standalone GN build can be
performed.

TESTED=Ran:
gn gen out/Release --args=is_debug=false
ninja -C out/Release

gn gen out/Debug --args=is_debug=true
ninja -C out/Debug

gn gen out/Debug --args="is_debug=true os=\"android\" "cpu_arch=\"arm\""
ninja -C out/Debug

gn gen out/Release --args="is_debug=false os=\"android\" "cpu_arch=\"arm\""
ninja -C out/Release

R=fbarchard@google.com

Review URL: https://webrtc-codereview.appspot.com/42289004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1317 16f28f9a-4ce2-e073-06de-1de4eb20be90