[VCSTARTUP] Implement (w)mainCRTStartup

__acrt_initialize is here as a stub to later support linking ucrt statically. When it is statically linked the real function should be called, but when the executable links to ucrtdll, the initialization happens when the DLL is loaded, so we call a stub here.

sdk/lib/vcruntime/__acrt_initialize_stub.cpp

@@ -0,0 +1,8 @@
+
+#include <internal_shared.h>
+
+extern "C"
+__crt_bool __cdecl __acrt_initialize()
+{
+    return true;
+}