[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/CMakeLists.txt

@@ -35,6 +35,8 @@ list(APPEND VCRT_STARTUP_SOURCES
     __acrt_initialize_stub.cpp
     __scrt_uninitialize_crt.cpp
     __vcrt_init_stubs.c
+    _onexit.c
+    atexit.c
     mainCRTStartup.cpp
     wmainCRTStartup.cpp
 )