lavc/aarch64: Add neon implementation for vsad8_intra

Provide optimized implementation for vsad8_intra function.

Performance comparison tests are shown below.
- vsad_5_c: 94.7
- vsad_5_neon: 20.7

Benchmarks and tests run with checkasm tool on AWS Graviton 3.

Signed-off-by: Hubert Mazur <hum@semihalf.com>
Signed-off-by: Martin Storsjö <martin@martin.st>