Provide optimized implementation for vsse_intra8 for arm64.

Performance tests are shown below.
- vsse_5_c: 87.7
- vsse_5_neon: 26.2

Benchmarks and tests are run with checkasm tool on AWS Graviton 3.
Co-authored-by: Martin Storsjö <martin@martin.st>
Signed-off-by: Martin Storsjö <martin@martin.st>