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
Merge pull request #4 from dgryski/amd64-core

Add amd64 optimized siphash core
Don't build asm version on appengine.
Build hash.go on appengine.