I had the same problem with one of my apps, which is how I ended up finding this. In my case, I uploaded two of my apps at the same time, using Xcode 7.1. One of the apps passed through processing within an hour. The other one was still in processing almost 24 hours later. To get past this issue, I created a new archive with an incremented build number, and uploaded it using the application loader. I did not turn off bitcode. The version that I uploaded using the application loader took less than 20 minutes to get through processing, and I've been able to submit my app for review. The version I submitted prior to this is still stuck in processing.
At least in the case of my app, using application loader appears to have solved the issue.