Add amd64 optimized siphash core

This code was produced by gcc from github.com/floodyberry/siphash

benchmark            old ns/op     new ns/op     delta
BenchmarkHash8       25.9          19.9          -23.17%
BenchmarkHash16      33.4          23.8          -28.74%
BenchmarkHash40      56.7          36.8          -35.10%
BenchmarkHash64      80.4          49.7          -38.18%
BenchmarkHash128     140           84.0          -40.00%
BenchmarkHash1K      1003          563           -43.87%
BenchmarkHash8K      7841          4331          -44.76%

benchmark            old MB/s     new MB/s     speedup
BenchmarkHash8       309.45       401.77       1.30x
BenchmarkHash16      479.34       673.41       1.40x
BenchmarkHash40      705.55       1087.64      1.54x
BenchmarkHash64      795.60       1286.88      1.62x
BenchmarkHash128     911.46       1523.03      1.67x
BenchmarkHash1K      1020.03      1817.62      1.78x
BenchmarkHash8K      1044.64      1891.28      1.81x