So, the problem is how to make “Sign in as different user” option available in personal action menu?
Solution: This is not a proper solution. But this will work till RTM of SP2013.  I need to change/update Welcome.ascx user control (Which is availble at ‘\15\TEMPLATE\CONTROLTEMPLATES\Welcome.ascx’)
Why this menu item is required :
This “Sign in as Different User” menu item is very useful when testing applications, but it can lead to problems especially when opening documents, say in Microsoft Word. So, it may be for these reasons that the option has been removed in SharePoint 2013.
You can add the menu item back in, but I would suggest only doing this on test or development SharePoint servers. To do this, repeat this edit on all servers in your SharePoint farm:

• Locate the file \15\TEMPLATE\CONTROLTEMPLATES\Welcome.ascx and open in a text editor.
• Add the following element before the existing element with the id of “ID_RequestAccess”:

          <SharePoint:MenuItemTemplate runat=”server” ID=”ID_LoginAsDifferentUser”
Text=”<%$Resources:wss,personalactions_loginasdifferentuser%>”
Description=”<%$Resources:wss,personalactions_loginasdifferentuserdescription%>”
MenuGroupId=”100″
Sequence=”100″
UseShortId=”true”
/>
After modifying Welcome.ascx (which is a non recommendation and a crappy solution!) you can see “Sign in as different user” option available in menu item.

The “Sign in as Different User” menu option has been done away with in SharePoint 2013. Microsoft has made it known that they have worked to make SharePoint 2013 a more streamlined, efficient, out-of-the-box solution, and in doing so are discouraging users from customization. However, some of the features that were removed still need to be navigated in some way. The “Sign in as Different User” command is one example.