Complete the ld.so boot cleanup: move most libc initialization from

_csu_finish() to _libc_preinit(), which is an .init_array function
in shared libc (and mark it INITFIRST) or a .preinit_array function
in static libc, grabbing the _dl_cb callback there from ld.so.  Then
in _csu_finish(), invoke the dl_clean_boot() callback to free ld.so's
startup bits before main() gets control.
Other cleanups this permits:
- move various startup structures into .data.rel.ro
- the dl* stubs in libc can be strong and call the callbacks provided
via _dl_cb
- no longer need to conditionalize dlctl() calls on presence of _DYNAMIC
original concept and full diff ok kettenis@
ok deraadt@

src/lib/libc/stdlib/thread_atexit.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: thread_atexit.c,v 1.1 2017/12/16 20:06:56 guenther Exp $ */
+/*	$OpenBSD: thread_atexit.c,v 1.2 2019/06/02 01:03:01 guenther Exp $ */
 /*
  * Copyright (c) 2017 Mark Kettenis <kettenis@openbsd.org>
  *
@@ -16,15 +16,11 @@
  */
 
 #include <dlfcn.h>
-#include <elf.h>
-#pragma weak _DYNAMIC
 #include <stdlib.h>
 #include <tib.h>
 
 #include "atexit.h"
 
-typeof(dlctl) dlctl asm("_dlctl") __attribute__((weak));
-
 __weak_alias(__cxa_thread_atexit, __cxa_thread_atexit_impl);
 
 int
@@ -37,8 +33,7 @@ __cxa_thread_atexit_impl(void (*func)(void *), void *arg, void *dso)
 	if (fnp == NULL)
 		return -1;
 
-	if (_DYNAMIC)
-		dlctl(NULL, DL_REFERENCE, dso);
+	dlctl(NULL, DL_REFERENCE, dso);
 
 	fnp->func = func;
 	fnp->arg = arg;