crypto/ecdsa: avoid needless ScalarBaseMult in s390x

crypto/ecdsa: avoid needless ScalarBaseMult in s390x

We are running the (slow on s390x) ScalarBaseMult and then discarding
the point because we are reusing randomPoint.

Copied the function 1:1 removing the point computation.

Change-Id: I6a6a46561633ab3bbbaef804481f6c5da15fe2fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/644775
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Roland Shoemaker <roland@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>