Provide optimized implementation of vsse16 for arm64.

Performance comparison tests are shown below.
- vsse_0_c: 257.7
- vsse_0_neon: 59.2

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