sched: make the current cpu pointer a member of the thread struct

With the current implementation, where the current cpu is a thread-local
variable, we cannot change the cpu pointer atomically during a context switch,
as we cannot obtain a pointer to a thread-local variable before the thread
has been run.

By moving the cpu pointer to the thread structure, we can change it atomically
by switching the thread-local base register (%fs on x64).