Cygwin: Fix signal handling issue introduced by PTY related change.

- After commit 4186409101, there is a
  regression in signal handling reported in
  https://www.cygwin.com/ml/cygwin/2019-10/msg00010.html. This patch
  fixes the issue.

winsup/cygwin/exceptions.cc

@@ -949,7 +949,7 @@ _cygtls::interrupt_setup (siginfo_t& si, void *handler, struct sigaction& siga)
   if (incyg)
     set_signal_arrived ();
 
-  if (!have_execed && ch_spawn.iscygwin ())
+  if (!have_execed && !(myself->exec_sendsig && !ch_spawn.iscygwin ()))
     proc_subproc (PROC_CLEARWAIT, 1);
   sigproc_printf ("armed signal_arrived %p, signal %d",
 		  signal_arrived, si.si_signo);