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    ...