crypto/sha256,crypto/sha512: make assembly structure consistent

crypto/sha256,crypto/sha512: make assembly structure consistent

Ensure separate implementations are implemented in different functions
called from Go, and that they can be turned off from a GODEBUG.

This will be necessary to test implementations separately for #69536.

Change-Id: I3e081deb7abb01b0665265e39c72fd4037dd48b3
Cq-Include-Trybots: luci.golang.try:gotip-linux-arm64-longtest,gotip-linux-amd64-longtest,gotip-linux-ppc64le_power8,gotip-linux-ppc64_power8
Reviewed-on: https://go-review.googlesource.com/c/go/+/614495
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>