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webrtc-audio-processing: refactor architecture dependency as a blind option

In preparation to the addition of a webrtc GStreamer plugin that will
have to select the webrtc-audio-processing package (and therefore
inherits its dependencies), this commits adds a
BR2_PACKAGE_WEBRTC_AUDIO_PROCESSING_ARCH_SUPPORTS blind option to avoid
duplicating the architecture dependencies of this package.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>