Fix way how function table is built. Allows to extend Exception class. (#210)

backend.native/tests/runtime/exceptions/extend0.kt

@@ -0,0 +1,9 @@
+class C : Exception("OK")
+
+fun main(args: Array<String>) {
+    try {
+        throw C()
+    } catch (e: Throwable) {
+        println(e.message!!)
+    }
+}