[release-branch.go1.23] 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 #71423
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>
(cherry picked from commit
6fc23a3cff5e38ff72923fee50f51254dcdc6e93)
Reviewed-on: https://go-review.googlesource.com/c/go/+/645515
Reviewed-by: Carlos Amedee <carlos@golang.org>