Provide optimized implementation for vsad_intra16 function for arm64.

Performance comparison tests are shown below.
- vsad_4_c: 177.5
- vsad_4_neon: 23.5

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