[VCSTARTUP] Implement atexit and _onexit

These must use a local table and cannot use the one from ucrtbase.
The table is initialized with a .CRT section initializer.

sdk/lib/vcruntime/atexit.c

@@ -0,0 +1,18 @@
+//
+// atexit.c
+//
+//      Copyright (c) 2024 Timo Kreuzer
+//
+// Implementation of atexit.
+//
+// SPDX-License-Identifier: MIT
+//
+
+#include <stdlib.h>
+
+int __cdecl atexit(void (__cdecl* _Func)(void))
+{
+    // Go through _onexit, so that the initializer is pulled in.
+    _onexit_t result = _onexit((_onexit_t)_Func);
+    return (result == NULL) ? -1 : 0;
+}