IIUC atexit isn't exported by ucrtbase, but still exists (maybe as a builtin) and is resolved to a module-local symbol which can be used to register functions executed on module process detach. It is called implicitly by C++ compilers to register static destructors.
-- v3: include: Define __cpuid(ex) as intrinsics when _MSC_VER is defined. include: Don't import atexit when building with ucrtbase. include: Guard rpcndr.h DECLSPEC_UUID definition.