Adds an assembly implementation of sha256.block for ppc64le to improve its
performance.  This implementation is largely based on the original amd64
implementation, which unrolls the 64 iterations of the inner loop.

Fixes golang#17652

benchmark               old ns/op     new ns/op     delta
BenchmarkHash8Bytes     1263          767           -39.27%
BenchmarkHash1K         14048         7766          -44.72%
BenchmarkHash8K         102245        55626         -45.60%

benchmark               old MB/s     new MB/s     speedup
BenchmarkHash8Bytes     6.33         10.43        1.65x
BenchmarkHash1K         72.89        131.85       1.81x
BenchmarkHash8K         80.12        147.27       1.84x

Change-Id: Ib4adf429423b20495580400be10bd7e171bcc70b
Reviewed-on: https://go-review.googlesource.com/32318
Reviewed-by: Carlos Eduardo Seo <cseo@linux.vnet.ibm.com>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>