multimedia/dav1d: work around crashes with clang >= 12 on i386

$ ffmpeg -hide_banner -i foo.y4m -c:v libsvtav1 -y foo.ivf
$ ffmpeg -hide_banner -i foo.ivf -f null /dev/null
[libdav1d @ 0x250c2c00] libdav1d 1.0.0
[libdav1d @ 0x250c2c00] Frame size limit reduced from 2147483647 to 67108864.
Bus error

* thread #1, name = 'ffmpeg', stop reason = signal SIGBUS: hardware error
    frame #0: 0x22e8c816 libdav1d.so.6`dav1d_data_wrap(buf=0x25100390,
ptr="\U00000012", sz=61259, free_callback=(libavcodec.so.58`libdav1d_data_free
at libdav1d.c:189), user_data=0x250d66c0) at lib.c:734
(lldb) bt
* thread #1, name = 'ffmpeg', stop reason = signal SIGBUS: hardware error
  * frame #0: 0x22e8c816 libdav1d.so.6`dav1d_data_wrap(buf=0x25100390,
ptr="\U00000012", sz=61259, free_callback=(libavcodec.so.58`libdav1d_data_free
at libdav1d.c:189), user_data=0x250d66c0) at lib.c:734
    frame #1: 0x213a4c33
libavcodec.so.58`libdav1d_receive_frame(c=<unavailable>, frame=<unavailable>) at
libdav1d.c:215:19
    frame #2: 0x21121082
libavcodec.so.58`decode_receive_frame_internal(avctx=0x250bfc00,
frame=<unavailable>) at decode.c:546:15