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

*** 2971,2980 ****
--- 2971,2981 ----
          scrwidth;
          semctl64;
          _semctl64;
          set_escaped_context_cleanup;
          set_setcontext_enforcement;
+         setcontext_sigmask;
          _setbufend;
          __set_errno;
          setprojrctl;
          _setregid;
          _setreuid;