crypto/ecdsa: fix s390x assembly with P-521

crypto/ecdsa: fix s390x assembly with P-521

I had incorrectly assumed that the blocksize was always the same as the
curve field size. This is true of P-256 and P-384, but not P-521.

Fixes #70660
Fixes #70771

Change-Id: Idb6b510fcd3dd42d9b1e6cf42c1bb92e0ce8bd07
Reviewed-on: https://go-review.googlesource.com/c/go/+/636015
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>