crt: Fix _onexit() function for DLL builds

MS documentation for _onexit function says:

  In the case when _onexit is called from within a DLL, routines registered
  with _onexit run when the DLL is unloaded, after DllMain is called with
  DLL_PROCESS_DETACH.

And for this functionality msvc statically links _onexit functionality for
DLL builds, and do not let applications to use msvcrt.dll's _onexit symbol
directly.

Fix this issue in mingw-w64 by redefining _onexit function to calls private
mingw-w64 atexit() function, which already distinguish between EXE and DLL
builds. Include this _onexit function into every CRT import library and
disable original _onexit symbol in all def files.

Signed-off-by: Martin Storsjö <martin@martin.st>