Provide short-circuit exit(3) implementation for rtld.

There is no atexit finalizers in rtld to call on rtld exiting (due to errors).

Submitted by:	bde
No objections from:	kan
MFC after:	2 weeks

libexec/rtld-elf/rtld.c

@@ -4364,6 +4364,17 @@ __getosreldate(void)
 	return (osreldate);
 }
 
+void
+exit(int status)
+{
+
+	_exit(status);
+}
+
+void (*__cleanup)(void);
+int __isthreaded = 0;
+int _thread_autoinit_dummy_decl = 1;
+
 /*
  * No unresolved symbols for rtld.
  */