GN: Build snapshot assembly files w/r/t current_toolchain.

runtime/bin/BUILD.gn

 # Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 import("gypi_contents.gni")
 import("../runtime_args.gni")
 import("../../build/compiled_action.gni")
 
 # Generate a resources.cc file for the service isolate without Observatory.
 action("gen_resources_cc") {
@@ skipping 494 matching lines @@
     deps = invoker.deps
     script = "../tools/bin_to_assembly.py"
     args = [
       "--input",
       rebase_path(invoker.input),
       "--output",
       rebase_path(invoker.output),
       "--symbol_name",
       invoker.symbol,
       "--target_os",
-      target_os,
+      current_os,
     ]
     if (invoker.executable) {
       args += [ "--executable" ]
     }
     inputs = [
       script,
       invoker.input,
     ]
     outputs = [
       invoker.output,
@@ skipping 394 matching lines @@
   ]
   if (is_linux || is_android) {
     cflags = [ "-fPIC" ]
   }
   if (is_win) {
     libs = [ "dart.lib" ]
     abs_root_out_dir = rebase_path(root_out_dir)
     ldflags = [ "/LIBPATH:$abs_root_out_dir" ]
   }
 }