[GN]: Support for loadable modules

tools/gn/example/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 executable("hello") {
   sources = [
     "hello.cc",
   ]
 
   deps = [
     ":hello_static",
     ":hello_shared",
+    ":hello_loadable",
   ]
 }
 
 shared_library("hello_shared") {
   sources = [
     "hello_shared.cc",
     "hello_shared.h",
   ]
 
   defines = [ "HELLO_SHARED_IMPLEMENTATION" ]
 }
 
+loadable_module("hello_loadable") {
+  sources = [
+    "hello_shared.cc",
+    "hello_shared.h",
+  ]
+}
+
 static_library("hello_static") {
   sources = [
     "hello_static.cc",
     "hello_static.h",
   ]
 }