libc.so can't be unloaded, so move the hidden atexit() and pthread_atfork()

stubs for the executable from crtbegin.o into libc, which lets them be
excluded from static links that don't use them.
For this, drop the normal crt{begin,end}S.o from libc.so: the .init and .fini
sections for libc aren't called at the right times anyway, so it's good that
they're unused.  libc.so just needs __guard_local and the .note.openbsd.ident
section, so add them to stack_protector.c for now (this will be improved)
"good time" deraadt@

src/lib/libc/stdlib/atexit.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: atexit.c,v 1.23 2015/10/25 18:03:17 guenther Exp $ */
+/*	$OpenBSD: atexit.c,v 1.24 2015/11/10 04:14:03 guenther Exp $ */
 /*
  * Copyright (c) 2002 Daniel Hartmeier
  * All rights reserved.
@@ -109,6 +109,18 @@ unlock:
 }
 DEF_STRONG(__cxa_atexit);
 
+/*
+ * Copy of atexit() used by libc and anything staticly linked into the
+ * executable.  This passes NULL for the dso, so the callbacks are only
+ * invoked by exit() and not dlclose()
+ */
+int
+atexit(void (*fn)(void))
+{
+	return (__cxa_atexit((void (*)(void *))fn, NULL, NULL));
+}
+DEF_STRONG(atexit);
+
 /*
  * Call all handlers registered with __cxa_atexit() for the shared
  * object owning 'dso'.