Forbid subclasses of NativePointed to have backing fields

(this crashes the compiler otherwise)

kotlin-native/backend.native/tests/compilerChecks/t60.kt

@@ -0,0 +1,10 @@
+import kotlinx.cinterop.*
+
+class Vertex constructor(rawPtr: NativePtr) : CStructVar(rawPtr) {
+    var x: Float = 0f
+    var y: Float = 0f
+    var r: Float = 0f
+    var g: Float = 0f
+    var b: Float = 0f
+    companion object : CStructVar.Type(40, 8)
+}