Use SIGUSR1 in OpenBSD as abort signal

Index: mono/utils/mono-threads-posix-signals.c
--- mono/utils/mono-threads-posix-signals.c.orig
+++ mono/utils/mono-threads-posix-signals.c
@@ -77,6 +77,8 @@ abort_signal_get (void)
 	if (abort_signum == -1)
 		abort_signum = mono_threads_suspend_search_alternative_signal ();
 	return abort_signum;
+#elif defined (__OpenBSD__)
+	return SIGUSR1;
 #elif defined (SIGTTIN)
 	return SIGTTIN;
 #else