crypto/sha256, cmd/internal/notsha256: improve PPC64 sha256

crypto/sha256, cmd/internal/notsha256: improve PPC64 sha256

This minimizes addi usage inside vector heavy loops. This
results in a small performance uptick on P9 ppc64le/linux.
Likewise, cleanup some minor whitespace issues around comments.

The implementation from crypto/sha256 is also shared with notsha256.
It is copied, but preserves notsha256's go:build directives. They are
otherwise identical now. Previously, bootstrap restrictions required
workarounds to support XXLOR on older toolchains. This is not needed
anymore as the minimum bootstrap (1.17) compiler will support XXLOR.

name               old speed      new speed      delta
Hash8Bytes/New     28.8MB/s ± 0%  30.5MB/s ± 0%  +5.98%
Hash8Bytes/Sum224  29.5MB/s ± 0%  31.3MB/s ± 0%  +6.17%
Hash8Bytes/Sum256  29.5MB/s ± 0%  31.2MB/s ± 0%  +5.80%
Hash1K/New          287MB/s ± 0%   312MB/s ± 0%  +8.60%
Hash1K/Sum224       289MB/s ± 0%   312MB/s ± 0%  +7.99%
Hash1K/Sum256       289MB/s ± 0%   312MB/s ± 0%  +7.98%
Hash8K/New          313MB/s ± 0%   338MB/s ± 0%  +8.12%
Hash8K/Sum224       313MB/s ± 0%   338MB/s ± 0%  +8.20%
Hash8K/Sum256       313MB/s ± 0%   338MB/s ± 0%  +8.12%

Change-Id: Ib386d6306673b4e6553ee745ec2e1b53a9722df1
Reviewed-on: https://go-review.googlesource.com/c/go/+/441815
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Archana Ravindar <aravind5@in.ibm.com>
Run-TryBot: Paul Murphy <murp@ibm.com>