OS-4470 lxbrand unblocking signals in new threads must be atomic

@@ -2971,10 +2971,11 @@
         scrwidth;
         semctl64;
         _semctl64;
         set_escaped_context_cleanup;
         set_setcontext_enforcement;
+        setcontext_sigmask;
         _setbufend;
         __set_errno;
         setprojrctl;
         _setregid;
         _setreuid;