crypto/sha256: adapt ppc64le asm to work on ppc64

Workaround the minor endian differences, and avoid needing to
stack a frame as extra VSRs can be used in a similar capacity.

The microbenchmarks show no significant differences on ppc64le/p9.

ppc64/linux performance difference on a POWER9:

name        old time/op    new time/op    delta
Hash8Bytes     686ns ± 0%     372ns ± 0%   -45.78%
Hash1K        9.17µs ± 0%    4.24µs ± 0%   -53.74%
Hash8K        67.9µs ± 0%    31.7µs ± 0%   -53.35%

Fixes #50785

Change-Id: I43d87670127df9767d54d10b5165b84e5b88f5d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/380776


Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Trust: Paul Murphy <murp@ibm.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>