crypto/internal/fips140/nistec: make p256NegCond constant time on ppc64le

crypto/internal/fips140/nistec: make p256NegCond constant time on ppc64le

Remove the branching instruction from p256NegCond which made it variable
time. The technique used matches that used in p256MovCond.

Fixes #71383
Fixes CVE-2025-22866

Change-Id: Ibc2a46814d856cbbdaf6cc0c5a415ed5d42ca793
Reviewed-on: https://go-review.googlesource.com/c/go/+/643735
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Paul Murphy <murp@ibm.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>