Add a memory barrier to the mutex initialization logic, try to work around

an issue reported by WebKit.

FossilOrigin-Name: 11a9a786ec06403addb47f5c6fb142b382fae522

src/mutex_unix.c

@@ -80,6 +80,18 @@ static int pthreadMutexNotheld(sqlite3_mutex *p){
 }
 #endif
 
+/*
+** Try to provide a memory barrier operation, needed for initialization only.
+*/
+void sqlite3MemoryBarrier(void){
+#if defined(__GNUC__)
+  __sync_synchronize();
+#endif
+#ifdef SQLITE_MEMORY_BARRIER
+  SQLITE_MEMORY_BARRIER;
+#endif
+}
+
 /*
 ** Initialize and deinitialize the mutex subsystem.
 */