Use native_arch for railgun sizes

Author: zeroSteiner

lib/rex/post/meterpreter/extensions/stdapi/railgun/dll.rb

@@ -119,7 +119,7 @@ def add_function(name, return_type, params, windows_name=nil, calling_conv="stdc
   def process_function_call(function, args, client)
     raise "#{function.params.length} arguments expected. #{args.length} arguments provided." unless args.length == function.params.length
 
-    if client.arch == ARCH_X64
+    if client.native_arch == ARCH_X64
       native = 'Q<'
     else
       native = 'V'