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

*** 152,161 ****
--- 152,163 ----
  
  extern ucontext_t *lx_syscall_regs(void);
  extern uintptr_t lx_find_brand_sp(void);
  extern const ucontext_t *lx_find_brand_uc(void);
  
+ extern void lx_jump_to_linux(ucontext_t *) __NORETURN;
+ 
  extern char *lx_fd_to_path(int fd, char *buf, int buf_size);
  extern int lx_lpid_to_spair(pid_t, pid_t *, lwpid_t *);
  extern int lx_lpid_to_spid(pid_t, pid_t *);
  
  extern void lx_ptrace_init();