Replace use of C11 atomics with atomic utils. This fixes
> error: address argument to atomic operation must be a pointer to a
> trivially-copyable type ('_Atomic(int) *' invalid)
error when compiling on AVR with LLVM.
Replace direct accesses to sched_active_thread and sched_active_pid with
the helper functions thread_getpid() and thread_get_active(). This serves
two purposes:
1. It makes accidental writes to those variable from outside core less likely.
2. Casting off the volatile qualifier is now well contained to those two
functions
