I had the same issue, here is how I got the menu back
- On the menubar go to View → Toolbars → Customize…
- Click on the “Commands” tab
- Select the “Toolbar” radio button and find “Standard” in the drop down list
- Click the “Add Command…” button
- Select the “Build” category
- Find and select the “Solution Platforms” command and click “OK”
- Move your new command to a comfortable place on your toolbar using “Move Up” and “Move Down”
- Enjoy not having to dig through Solution Properties to change the platform