crypto/sha{256,512}: unname result parameters for consistency

Sum224 and Sum256 didn't look the same at:

    https://golang.org/pkg/crypto/sha256/

Now they match. Likewise with sha512's funcs.

Per:
https://github.com/golang/go/wiki/CodeReviewComments#named-result-parameters

Change-Id: I6b88c8ef15141c78a6cddeb0960b3ad52db34244
Reviewed-on: https://go-review.googlesource.com/c/go/+/322329


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Brad Fitzpatrick <bradfitz@golang.org>
Trust: Katie Hockman <katie@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>