gstreamer1.0-libav breaks video playback

gstreamer1.0-libav is supposed to make video playback faster and supporting more formats by using libav but that's broken on aarch64:

e.g. when playing videos in epiphany:

(WebKitWebProcess:1957): GStreamer-WARNING **: 10:37:41.690: Failed to load plugin '/usr/lib/aarch64-linux-gnu/gstreamer-1.0/libgstlibav.so': /lib/aarch64-linux-gnu/libgomp.so.1: cannot allocate memory in static TLS block

this can be worked around by:

LD_PRELOAD=/lib/aarch64-linux-gnu/libgomp.so.1 epiphany

which gives okish video speed.