MINOR: threads: add a portable barrier for threads and non-threads

HA_BARRIER() is just a simple memory barrier to prevent the compiler
from reordering our code.
diff --git a/include/common/hathreads.h b/include/common/hathreads.h
index 25c1737..3524ef1 100644
--- a/include/common/hathreads.h
+++ b/include/common/hathreads.h
@@ -60,6 +60,7 @@
 		*(val);							\
 	})
 
+#define HA_BARRIER() do { } while (0)
 
 #define THREAD_SYNC_INIT(m)  do { /* do nothing */ } while(0)
 #define THREAD_SYNC_ENABLE() do { /* do nothing */ } while(0)
@@ -118,6 +119,8 @@
 		(*val);							\
 	})
 
+#define HA_BARRIER() pl_barrier()
+
 #define THREAD_SYNC_INIT(m)   thread_sync_init(m)
 #define THREAD_SYNC_ENABLE()  thread_sync_enable()
 #define THREAD_WANT_SYNC()    thread_want_sync()