truehd: add hand-scheduled ARM asm version of mlp_filter_channel.

Profiling results for overall audio decode and the mlp_filter_channel(_arm)
function in particular are as follows:

              Before          After
              Mean   StdDev   Mean   StdDev  Confidence  Change
6:2 total     380.4  22.0     370.8  17.0    87.4%       +2.6%  (insignificant)
6:2 function  60.7   7.2      36.6   8.1     100.0%      +65.8%
8:2 total     357.0  17.5     343.2  19.0    97.8%       +4.0%  (insignificant)
8:2 function  60.3   8.8      37.3   3.8     100.0%      +61.8%
6:6 total     717.2  23.2     658.4  15.7    100.0%      +8.9%
6:6 function  140.4  12.9     81.5   9.2     100.0%      +72.4%
8:8 total     981.9  16.2     896.2  24.5    100.0%      +9.6%
8:8 function  193.4  15.0     103.3  11.5    100.0%      +87.2%

Experiments with adding preload instructions to this function yielded no
useful benefit, so these have not been included.

The assembly version has also been tested with a fuzz tester to ensure that
any combinations of inputs not exercised by my available test streams still
generate mathematically identical results to the C version.
truehd: break out part of rematrix_channels into platform-specific callback.

Verified with profiling that this doesn't have a measurable effect upon
overall performance.
truehd: add hand-scheduled ARM asm version of ff_mlp_rematrix_channel.

Profiling results for overall audio decode and the rematrix_channels function
in particular are as follows:

              Before          After
              Mean   StdDev   Mean   StdDev  Confidence  Change
6:2 total     370.8  17.0     348.8  20.1    99.9%       +6.3%
6:2 function  46.4   8.4      45.8   6.6     18.0%       +1.2%  (insignificant)
8:2 total     343.2  19.0     339.1  15.4    54.7%       +1.2%  (insignificant)
8:2 function  38.9   3.9      40.2   6.9     52.4%       -3.2%  (insignificant)
6:6 total     658.4  15.7     604.6  20.8    100.0%      +8.9%
6:6 function  109.0  8.7      59.5   5.4     100.0%      +83.3%
8:8 total     896.2  24.5     766.4  17.6    100.0%      +16.9%
8:8 function  223.4  12.8     93.8   5.0     100.0%      +138.3%

The assembly version has also been tested with a fuzz tester to ensure that
any combinations of inputs not exercised by my available test streams still
generate mathematically identical results to the C version.
truehd: tune VLC decoding for ARM.

Profiling on a Raspberry Pi revealed the best performance to correspond
with VLC_BITS = 5. Results for overall audio decode and the get_vlc2 function
in particular are as follows:

              Before          After
              Mean   StdDev   Mean   StdDev  Confidence  Change
6:2 total     348.8  20.1     339.6  15.1    88.8%       +2.7%  (insignificant)
6:2 function  38.1   8.1      26.4   4.1     100.0%      +44.5%
8:2 total     339.1  15.4     324.5  15.5    99.4%       +4.5%
8:2 function  33.8   7.0      27.3   5.6     99.7%       +23.6%
6:6 total     604.6  20.8     572.8  20.6    100.0%      +5.6%
6:6 function  95.8   8.4      68.9   8.2     100.0%      +39.1%
8:8 total     766.4  17.6     741.5  21.2    100.0%      +3.4%
8:8 function  106.0  11.4     86.1   9.9     100.0%      +23.1%
truehd: break out part of output_data into platform-specific callback.

Verified with profiling that this doesn't have a measurable effect upon
overall performance.
truehd: add hand-scheduled ARM asm version of ff_mlp_pack_output.

Profiling results for overall decode and the output_data function in
particular are as follows:

              Before          After
              Mean   StdDev   Mean   StdDev  Confidence  Change
6:2 total     339.6  15.1     329.3  16.0    95.8%       +3.1%  (insignificant)
6:2 function  24.6   6.0      9.9    3.1     100.0%      +148.5%
8:2 total     324.5  15.5     323.6  14.3    15.2%       +0.3%  (insignificant)
8:2 function  20.4   3.9      9.9    3.4     100.0%      +104.7%
6:6 total     572.8  20.6     539.9  24.2    100.0%      +6.1%
6:6 function  54.5   5.6      16.0   3.8     100.0%      +240.9%
8:8 total     741.5  21.2     702.5  18.5    100.0%      +5.6%
8:8 function  63.9   7.6      18.4   4.8     100.0%      +247.3%

The assembly version has also been tested with a fuzz tester to ensure that
any combinations of inputs not exercised by my available test streams still
generate mathematically identical results to the C version.