vendor: update buildkit to 2943a0838

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

vendor/golang.org/x/crypto/poly1305/sum_ppc64le.go

@@ -9,17 +9,6 @@ package poly1305
 //go:noescape
 func update(state *macState, msg []byte)
 
-func sum(out *[16]byte, m []byte, key *[32]byte) {
-	h := newMAC(key)
-	h.Write(m)
-	h.Sum(out)
-}
-
-func newMAC(key *[32]byte) (h mac) {
-	initialize(key, &h.r, &h.s)
-	return
-}
-
 // mac is a wrapper for macGeneric that redirects calls that would have gone to
 // updateGeneric to update.
 //