Adds an assembly implementation of sha512.block for ppc64le to improve its
performance.  This implementation is largely based on the original amd64
implementation, unrolling the 80 iterations of the inner loop.

Fixes #17660

benchmark               old ns/op     new ns/op     delta
BenchmarkHash8Bytes     1715          1133          -33.94%
BenchmarkHash1K         10098         5513          -45.41%
BenchmarkHash8K         68004         35278         -48.12%

benchmark               old MB/s     new MB/s     speedup
BenchmarkHash8Bytes     4.66         7.06         1.52x
BenchmarkHash1K         101.40       185.72       1.83x
BenchmarkHash8K         120.46       232.21       1.93x

Change-Id: Ifd55a49a24cb159b3a09a8e928c3f37727aca103
Reviewed-on: https://go-review.googlesource.com/32320Reviewed-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>