audio/webrtc-audio-processing: mark broken on powerpc*

In file included from ../webrtc/system_wrappers/source/cpu_features.cc:13:
../webrtc/rtc_base/system/arch.h:54:2: error: Please add support for your architecture in rtc_base/system/arch.h
#error Please add support for your architecture in rtc_base/system/arch.h

^

../webrtc/rtc_base/system/arch.h:58:2: error: Define either WEBRTC_ARCH_LITTLE_ENDIAN or WEBRTC_ARCH_BIG_ENDIAN
#error Define either WEBRTC_ARCH_LITTLE_ENDIAN or WEBRTC_ARCH_BIG_ENDIAN

^

2 errors generated.

Reported by: pkg-fallout