webrtc-audio-processing: disable unsupported target architectures

Currently, webrtc-audio-processing only provides support for the following
architectures (see src/typedefs.h from the source directory):
- x86_64
- i*86
- armel

Fixes http://autobuild.buildroot.org/results/ff777bb06eddafa71327015e1d39fe911c1439d8



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>