cmd/internal/notsha256: add purego tag as needed

cmd/internal/notsha256: add purego tag as needed

This permits building the package with gccgo, when using gccgo
as a bootstrap compiler.

Fixes #53662

Change-Id: Ic7ae9323ec5954e9306a32e1160e9aa1ed3aa202
Reviewed-on: https://go-review.googlesource.com/c/go/+/415935
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: David Chase <drchase@google.com>