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.

headers/private/libroot/pthread_private.h

@@ -50,12 +50,12 @@ typedef struct _pthread_rwlockattr {
 typedef void (*pthread_key_destructor)(void *data);
 
 struct pthread_key {
-	vint32		sequence;
+	int32		sequence;
 	pthread_key_destructor destructor;
 };
 
 struct pthread_key_data {
-	vint32		sequence;
+	int32		sequence;
 	void		*value;
 };