crypto/sha256: block implementation in amd64 assembly

Benchmark on Intel(R) Xeon(R) CPU X5650  @ 2.67GHz

benchmark              old ns/op    new ns/op    delta
BenchmarkHash8Bytes         1259          677  -46.23%
BenchmarkHash1K            14387         6749  -53.09%
BenchmarkHash8K           106006        50107  -52.73%

benchmark               old MB/s     new MB/s  speedup
BenchmarkHash8Bytes         6.35        11.81    1.86x
BenchmarkHash1K            71.17       151.72    2.13x
BenchmarkHash8K            77.28       163.49    2.12x

R=agl
CC=golang-dev
https://golang.org/cl/28460043