Remove mingw-w64 version of _fmode. _fmode should not be linked directly into the executable. Instead, the export from msvcrt should be used. On Windows 10 it's defined to *__p__fmode(), which is exported from ucrtbase.dll
Also _fmode in msvcrt.dll is initialized with 0.
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.
We previously used 2 different versions: one from wine and one from mingw-w64.
The former was used in msvcrt, the latter was statically compiled into the executable. When using MS libs, there is only one _matherr, which is statically linked into the executable and does nothing (it's not really a function for users to be called).
_setusermatherror should only exist in msvcrt and not statically, which wouldn't work at all.
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.