crypto/md5,crypto/sha1: apply fips140=only to Write and Sum, not New

crypto/md5,crypto/sha1: apply fips140=only to Write and Sum, not New

New is called to get a Hash which can then be rejected with an error
(instead of a panic) from fips140only.ApprovedHash.

Also, it's reasonable to call New().Size() and then not use the hash.

Change-Id: I6a6a4656c43528d169c4b28c8b6de48448236d4f
Reviewed-on: https://go-review.googlesource.com/c/go/+/641317
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>