ffmpeg: actually use gas-preprocessor on ios
ffmpeg: actually use gas-preprocessor on ios

Also pass the necessary flags for iphone minimum target version
selection so that object files are built for the correct version.  This
may be important if some newer code/functionality cannot be used on
older devices due to hardware or software limitations.

Part-of: <https://gitlab.freedesktop.org/gstreamer/cerbero/-/merge_requests/715>

nirbheek/cerbero,nirbheek/cerbero,nirbheek/cerbero,nirbheek/cerbero