Use FPU interrupt to determine if userspace thread is using SSE

[4e554c4c]

ThePowersGang gave me this idea on #osdev, it'll probably be a long time until we use it but all the better reason to write it down.

I'll use this issue now to ask a question: Do you think that I should continue to create issues like this with milestones or should we create something like a TODO issue with a checklist that can be updated? We could also begin to use the new "Projects" functionality of github to organize the issues if they get out of hand.