System module format - fixes function hoisting failure case (#8820)

* failing test case

* fix function hoist bug

packages/babel-plugin-transform-modules-systemjs/test/fixtures/systemjs/export-fn-decl/input.mjs

@@ -0,0 +1,10 @@
+var testProp = 'test property';
+
+function testFunc() {
+  return 'test function';
+}
+
+export {
+  testFunc,
+  testProp
+};

packages/babel-plugin-transform-modules-systemjs/test/fixtures/systemjs/export-fn-decl/output.mjs

@@ -0,0 +1,18 @@
+System.register([], function (_export, _context) {
+  "use strict";
+
+  var testProp;
+
+  function testFunc() {
+    return 'test function';
+  }
+
+  _export("testFunc", testFunc);
+
+  return {
+    setters: [],
+    execute: function () {
+      _export("testProp", testProp = 'test property');
+    }
+  };
+});