crypto/sha256,crypto/sha512: improve performance for sha{256,512}.block on ppc64le

This updates sha256.block and sha512.block to use vector instructions.  While
each round must still be performed independently, this allows for the use of
the vshasigma{w,d} crypto acceleration instructions.

For crypto/sha256:

benchmark               old ns/op     new ns/op     delta
BenchmarkHash8Bytes     570           300           -47.37%
BenchmarkHash1K         7529          3018          -59.91%
BenchmarkHash8K         55308         21938         -60.33%

benchmark               old MB/s     new MB/s     speedup
BenchmarkHash8Bytes     14.01        26.58        1.90x
BenchmarkHash1K         136.00       339.23       2.49x
BenchmarkHash8K         148.11       373.40       2.52x

For crypto/sha512:

benchmark               old ns/op     new ns/op     delta
BenchmarkHash8Bytes     725           394           -45.66%
BenchmarkHash1K         5062          2107          -58.38%
BenchmarkHash8K         34711         13918         -59.90%

benchmark               old MB/s     new MB/s     speedup
BenchmarkHash8Bytes     11.03        20.29        1.84x
BenchmarkHash1K         202.28       485.84       2.40x
BenchmarkHash8K         236.00       588.56       2.49x

Fixes #20069

Change-Id: I28bffe6e9eb484a83a004116fce84acb4942abca
Reviewed-on: https://go-review.googlesource.com/41391
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Carlos Eduardo Seo <cseo@linux.vnet.ibm.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>