Neon version of rftfsub_128()

The performance gain on a Nexus 7 reported by audioproc is ~3.3%

The output is bit exact.

BUG=3131
TESTED=trybots and manually on N7
R=bjornv@webrtc.org, cd@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/14819004

Patch from Scott LaVarnway <slavarnw@gmail.com>.

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6628 4adac7df-926f-26a2-2b94-8c16560cd09d