Provide optimized implementation of vsad16 function for arm64.

Performance comparison tests are shown below.
- vsad_0_c: 285.2
- vsad_0_neon: 39.5

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