exceptor: Add SA_ONSTACK flag the POSIX backend (#802)

Add the SA_ONSTACK flag to handle situations where an alternative
signal stack has been configured using sigaltstack(). In the situations
where no alternative stack was configured, this behaves as if the flag
has not been set.

gum/backend-posix/gumexceptor-posix.c

@@ -253,6 +253,9 @@ gum_exceptor_backend_attach (GumExceptorBackend * self)
   action.sa_sigaction = gum_exceptor_backend_on_signal;
   sigemptyset (&action.sa_mask);
   action.sa_flags = SA_SIGINFO | SA_NODEFER;
+#ifdef SA_ONSTACK
+  action.sa_flags |= SA_ONSTACK;
+#endif
   for (i = 0; i != G_N_ELEMENTS (handled_signals); i++)
   {
     gint sig = handled_signals[i];