pluginloader: make sure gst-plugin-scanner is called with the right arch on OSX

pluginloader: make sure gst-plugin-scanner is called with the right arch on OSX

On OSX, GStreamer might be built as a 'fat/universal' binary containing
both 32-bit and 64-bit code. We must take care that gst-plugin-scanner
is executed with the same architecture as the GStreamer core, otherwise
bad things may happen and core/scanner will not be able to communicate
properly.

Should fix issues with (32-bit) firefox using a 32-bit GStreamer core
which then spawns a 'universal' gst-plugin-scanner binary which gets
run in 64-bit mode, causing 100% cpu usage / busy loops or just hanging
firefox until killed.

https://bugzilla.gnome.org/show_bug.cgi?id=615357