arm: make ff_mlp_filter_channel_arm and ff_mlp_rematrix_channel_arm position independent

No significant difference in used cpu cycles on a cortex-a9.
arm: mlpdsp: handle pic offset calculation in a macro

Makes the code easier to read since it hides different offset
calculations for arm and thumb mode.
fate: add dolby true hd tests

The existing meridian audio test does not test
ff_mlp_rematrix_channel_arm. This sample (first 640k of
https://samples.libav.org/A-codecs/TrueHD/TrueHD.raw) uses
ff_mlp_rematrix_channel_arm. Since this sample has 5.1 channels it also
allows testing the integrated downmixing.