common_audio/signal_processing: Fixes arm compilation issues with gcc 4.8

In r6240 gcc was rolled from 4.6 to 4.8 changing the behavior on arm. The output of ComplexFFT differs causing both AECM and NS to perform worse. Looking at issues on gcc it says that there could be a memory shuffling/optimization despite using volatile affecting the output.
Splitting the three instructions in one call into two separate calls makes the compiler take proper actions resulting in correct outputs.

BUG=3370,3395
TESTED=trybots
R=kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/21549004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6261 4adac7df-926f-26a2-2b94-8c16560cd09d