Build libaom with CONFIG_LOWBITDEPTH=1

[Kagami]

See: https://bugs.chromium.org/p/aomedia/issues/detail?id=2062 and https://forum.doom9.org/showpost.php?p=1852393&postcount=995

CONFIG_LOWBITDEPTH=0 (default)

$ time aomenc blue_sky_360p_60f.y4m -o 1.webm --limit=10 --passes=1 --cpu-used=4 --end-usage=q --cq-level=25 --tile-columns=3 --threads=8
Pass 1/1 frame   10/0          0B   24451 us 408.98 fps [ETA  unknown]   61757F  20751F   3881F   2584F   5778F   1704F   2248F
Pass 1/1 frame   10/10    102194B   81755b/f 2043875b/s   23731 ms (0.42 fps)
224% cpu 23.931 total

CONFIG_LOWBITDEPTH=1

$ time aomenc blue_sky_360p_60f.y4m -o 2.webm --limit=10 --passes=1 --cpu-used=4 --end-usage=q --cq-level=25 --tile-columns=3 --threads=8
Pass 1/1 frame   10/0          0B   12706 us 787.03 fps [ETA  unknown]   61757F  20751F   3881F   2584F   5778F   1704F   2248F
Pass 1/1 frame   10/10    102194B   81755b/f 2043875b/s   15530 ms (0.64 fps)
238% cpu 15.725 total

[Kagami]

Enabled in latest media-autobuild_suite.