Add experimental 128-bit siphash output

The changes required for 128-bit output are small, so hash128.go is a
modified copy of hash.go.  I've also included an amd64 assembly core,
similarly modified from the amd64 core for the regular 64-bit siphash.
Patching the assembly core was a bit trickier.

The test cases for siphash-128 are from the reference implementation.

Benchmarks for the pure-Go vs the asm core:

benchmark                old ns/op     new ns/op     delta
BenchmarkHash128_8       35.0          29.0          -17.14%
BenchmarkHash128_16      42.2          33.3          -21.09%
BenchmarkHash128_40      65.5          46.5          -29.01%
BenchmarkHash128_64      88.2          60.0          -31.97%
BenchmarkHash128_128     150           95.1          -36.60%
BenchmarkHash128_1K      1023          576           -43.70%
BenchmarkHash128_8K      7953          4357          -45.22%

benchmark                old MB/s     new MB/s     speedup
BenchmarkHash128_8       228.56       275.44       1.21x
BenchmarkHash128_16      379.05       480.80       1.27x
BenchmarkHash128_40      610.29       860.14       1.41x
BenchmarkHash128_64      725.90       1066.54      1.47x
BenchmarkHash128_128     850.00       1346.35      1.58x
BenchmarkHash128_1K      1000.77      1777.15      1.78x
BenchmarkHash128_8K      1029.93      1879.78      1.83x