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

*** 496,505 ****
--- 496,526 ----
                  lx_err_fatal("Unable to initialize thread-specific data: %s",
                      strerror(err));
          }
  }
  
+ void
+ lx_jump_to_linux(ucontext_t *ucp)
+ {
+         extern void setcontext_sigmask(ucontext_t *);
+ 
+         /*
+          * Call into this private libc interface to allow us to use only the
+          * signal mask handling part of a regular setcontext() operation.
+          */
+         setcontext_sigmask(ucp);
+ 
+         if (syscall(SYS_brand, B_JUMP_TO_LINUX, ucp) != 0) {
+                 lx_err_fatal("B_JUMP_TO_LINUX failed: %s", strerror(errno));
+         }
+ 
+         /*
+          * This system call should not return.
+          */
+         abort();
+ }
+ 
  static void
  lx_start(uintptr_t sp, uintptr_t entry)
  {
          ucontext_t jump_uc;
  
*** 528,546 ****
           */
          LX_REG(&jump_uc, REG_RDX) = NULL;
  #endif
  
          lx_debug("starting Linux program sp %p ldentry %p", sp, entry);
! 
!         /*
!          * This system call should not return.
!          */
!         if (syscall(SYS_brand, B_JUMP_TO_LINUX, &jump_uc) == -1) {
!                 lx_err_fatal("B_JUMP_TO_LINUX failed: %s",
!                     strerror(errno));
!         }
!         abort();
  }
  
  /*ARGSUSED*/
  int
  lx_init(int argc, char *argv[], char *envp[])
--- 549,559 ----
           */
          LX_REG(&jump_uc, REG_RDX) = NULL;
  #endif
  
          lx_debug("starting Linux program sp %p ldentry %p", sp, entry);
!         lx_jump_to_linux(&jump_uc);
  }
  
  /*ARGSUSED*/
  int
  lx_init(int argc, char *argv[], char *envp[])