[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/CMakeLists.txt

@@ -31,7 +31,10 @@ list(APPEND VCRT_RUNTIME_SOURCES
 )
 
 list(APPEND VCRT_STARTUP_SOURCES
+    __acrt_initialize_stub.cpp
     __scrt_uninitialize_crt.cpp
+    mainCRTStartup.cpp
+    wmainCRTStartup.cpp
 )
 
 if(${ARCH} STREQUAL "i386")