MINOR: thread: provide a set of lock initialisers

This patch adds ha_spin_init() and ha_rwlock_init() which are used as
a callback to initialise locks at boot time. They perform exactly the
same as HA_SPIN_INIT() or HA_RWLOCK_INIT() but from within a real
function.
diff --git a/include/common/hathreads.h b/include/common/hathreads.h
index d2fd400..c5be9b2 100644
--- a/include/common/hathreads.h
+++ b/include/common/hathreads.h
@@ -948,6 +948,9 @@
 #define __ha_barrier_full __sync_synchronize
 #endif
 
+void ha_spin_init(HA_SPINLOCK_T *l);
+void ha_rwlock_init(HA_RWLOCK_T *l);
+
 #endif /* USE_THREAD */
 
 static inline void __ha_compiler_barrier(void)