kernel: User{Event, Timer}: Use atomic access where necessary
kernel: atomic_*() functions rework

 * No need for the atomically changed variables to be declared as
   volatile.
 * Drop support for atomically getting and setting unaligned data.
 * Introduce atomic_get_and_set[64]() which works the same as
   atomic_set[64]() used to. atomic_set[64]() does not return the
   previous value anymore.
kernel: Improve spinlock implementation

atomic_or() and atomic_and() are not supported by x86 are need to be
emulated using CAS. Use atomic_get_and_set() and atomic_set() instead.
docs: Update atomic_*() functions documentation