kernel: Fix acquire_read_spinlock() acquire checks

If the initial attempt to acquire read spinlock fails we use more relaxed
loop (which doesn't require CPU to lock the bus). However, check in that
loop, incorrectly, didn't allow a lock to be acquired when there was at
least one other reader.
kernel: Relax atomic loads in SMP code

The main purpose of using atomic_get() was the necessity of a compiler
barrier to prevent the compiler from optimizing busy loops. However,
each such loop contains in its body at least one statement that acts
as a compiler barrier (namely, cpu_wait() or cpu_pause()) making
atomic_get() redundant (well, atomic_get() is stronger - it also issues
a load barrier but in these particular cases we do not need it).