Land rapid7#7199, bcook's pr for is_system fix

fixes issue that caused is_system to crash meterp

Author: David Maloney

lib/msf/base/sessions/meterpreter.rb

@@ -326,9 +326,7 @@ def update_session_info
     username = self.sys.config.getuid
     sysinfo  = self.sys.config.sysinfo
 
-    self.platform =
-      self.sys.config.sysinfo["Architecture"].downcase + '/' +
-      self.platform.split('/')[0] +'/' +
+    self.platform = self.platform.split('/')[0] + '/' +
       case self.sys.config.sysinfo['OS']
       when /windows/i
         Msf::Module::Platform::Windows