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

@@ -1555,13 +1555,13 @@
         /*
          * These variables must be "volatile", as they are modified after the
          * getcontext() stores the register state:
          */
         volatile boolean_t signal_delivered = B_FALSE;
-        volatile uintptr_t lxfp;
-        volatile uintptr_t old_tsd_sp;
-        volatile int newstack;
+        volatile uintptr_t lxfp = 0;
+        volatile uintptr_t old_tsd_sp = 0;
+        volatile int newstack = 0;
 
         /*
          * This function involves modifying the Linux process stack for this
          * thread.  To do so without corruption requires us to exclude other
          * signal handlers (or emulated system calls called from within those

@@ -1803,17 +1803,15 @@
                 LX_REG(&jump_uc, REG_RDI) = hargs[0];
                 LX_REG(&jump_uc, REG_RSI) = hargs[1];
                 LX_REG(&jump_uc, REG_RDX) = hargs[2];
 #endif
 
-                if (syscall(SYS_brand, B_JUMP_TO_LINUX, &jump_uc) == -1) {
-                        lx_err_fatal("B_JUMP_TO_LINUX failed: %s",
-                            strerror(errno));
+                lx_jump_to_linux(&jump_uc);
                 }
-        }
 
         assert(0);
+        abort();
 
 after_signal_handler:
         /*
          * Ensure all nested signal handlers have completed correctly
          * and then remove our stack reservation.