add port of field operations assembler from libsecp256k1

scalar_generic.go

@@ -0,0 +1,18 @@
+//go:build !amd64
+
+package p256k1
+
+// Generic stub implementations for non-AMD64 architectures.
+// These simply forward to the pure Go implementations.
+
+func scalarMulAVX2(r, a, b *Scalar) {
+	r.mulPureGo(a, b)
+}
+
+func scalarAddAVX2(r, a, b *Scalar) {
+	r.addPureGo(a, b)
+}
+
+func scalarSubAVX2(r, a, b *Scalar) {
+	r.subPureGo(a, b)
+}