Add an Armv8.4-A DotProd code path for standard bitdepth convolutions.
Only horizontal-vertical (HV) convolutions have 6-tap specialisations
of their vertical passes. All other convolutions are 4- or 8-tap
filters which fit well with the 4-element SDOT instruction.

Benchmarks show up-to 7-29% FPS increase depending on the input video
and the CPU used.

This patch will increase the .text by around 6.5 KiB.

Performance highly depends on the SDOT and MLA throughput ratio, this
can be seen on the vertical filter cases. Small cores are also
affected by the TBL execution latencies:

Relative performance to the C reference on some CPUs:

                          A76      A78       X1      A55
regular w4 hv neon:      5.52x    5.78x   10.75x    8.27x
regular w4 hv dotprod:   7.94x    8.49x   16.84x    8.09x
sharp w4 hv neon:        5.27x    5.22x    9.06x    7.87x
sharp w4 hv dotprod:     6.61x    6.73x   12.64x    6.89x

regular w8 hv neon:      1.95x    2.19x    2.56x    3.16x
regular w8 hv dotprod:   3.23x    2.81x    3.20x    3.26x
sharp w8 hv neon:        1.61x    1.79x    2.05x    2.72x
sharp w8 hv dotprod:     2.72x    2.29x    2.66x    2.76x

regular w16 hv neon:     1.63x    2.04x    2.16x    2.73x
regular w16 hv dotprod:  2.72x    2.57x    2.67x    2.80x
sharp w16 hv neon:       1.33x    1.67x    1.74x    2.34x
sharp w16 hv dotprod:    2.31x    2.14x    2.26x    2.39x

regular w32 hv neon:     1.48x    1.92x    1.94x    2.51x
regular w32 hv dotprod:  2.49x    2.40x    2.33x    2.58x
sharp w32 hv neon:       1.21x    1.56x    1.53x    2.14x
sharp w32 hv dotprod:    2.12x    2.02x    2.00x    2.22x

regular w64 hv neon:     1.42x    1.87x    1.85x    2.40x
regular w64 hv dotprod:  2.40x    2.32x    2.21x    2.46x
sharp w64 hv neon:       1.16x    1.52x    1.46x    2.04x
sharp w64 hv dotprod:    2.02x    1.96x    1.90x    2.11x

regular w128 hv neon:    1.39x    1.84x    1.80x    2.27x
regular w128 hv dotprod: 2.33x    2.28x    2.14x    2.35x
sharp w128 hv neon:      1.14x    1.50x    1.42x    1.94x
sharp w128 hv dotprod:   1.98x    1.93x    1.84x    2.03x

regular w8 h neon:       2.61x    3.20x    3.51x    3.55x
regular w8 h dotprod:    4.43x    5.17x    6.26x    4.30x
sharp w8 h neon:         2.01x    2.80x    2.89x    3.12x
sharp w8 h dotprod:      4.42x    5.16x    6.27x    4.28x

regular w16 h neon:      2.17x    3.13x    2.92x    3.35x
regular w16 h dotprod:   4.38x    4.27x    4.53x    3.90x
sharp w16 h neon:        1.74x    2.65x    2.48x    2.92x
sharp w16 h dotprod:     4.33x    4.27x    4.53x    3.91x

regular w64 h neon:      1.92x    2.82x    2.39x    2.96x
regular w64 h dotprod:   3.68x    3.60x    3.40x    3.18x
sharp w64 h neon:        1.47x    2.33x    2.05x    2.54x
sharp w64 h dotprod:     3.68x    3.60x    3.40x    3.17x

regular w4 v neon:       5.39x    7.38x   10.27x   11.41x
regular w4 v dotprod:    9.46x   14.15x   18.72x    9.84x
sharp w4 v neon:         4.51x    6.39x    8.17x   10.70x
sharp w4 v dotprod:      9.35x   14.20x   18.63x    9.78x

regular w16 v neon:      3.03x    4.03x    4.65x    6.28x
regular w16 v dotprod:   4.64x    3.75x    4.78x    3.89x
sharp w16 v neon:        2.29x    3.09x    3.44x    5.52x
sharp w16 v dotprod:     4.62x    3.74x    4.77x    3.89x

regular w64 v neon:      2.17x    3.14x    3.19x    4.46x
regular w64 v dotprod:   3.43x    3.00x    3.31x    2.74x
sharp w64 v neon:        1.61x    2.42x    2.34x    3.89x
sharp w64 v dotprod:     3.38x    3.00x    3.29x    2.73x