This "initialization" copies the value of the local variable to the one exported from msvcrt, which is the same. Also there shouldn't be a local version of the variable in the first place.
This removes the broken wine version of atexit and onexit. It keeps only dllonexit, which is implemented properly. The previous __call_atexit is moved to where the mingw onexit/atexit code is and adjusts it to work with the existing code. A call to __call_atexit is added in __tmainCRTStartup after the main function was called.
Native CRT on ARM & ARM64 Windows doesn't have these functions.
For compatibility, it's mandatory to not have it at all. Otherwise,
ARM executables built for ReactOS do not run on true ARM Windows.
Consider doing full sync with mingw-w64 CRT in future.