tweak `atexit` to get rid of malloc

[fenugrec]

Originally posted by @fenugrec in #128 (comment)

See comment above;
in certain cases the builtint atexit implementation pulls in malloc and some other stuff.

More information at https://stackoverflow.com/questions/48711221/how-to-prevent-inclusion-of-c-library-destructors-and-atexit

[fenugrec]

-fno-use-cxa-atexit will not work :
command-line option '-fno-use-cxa-atexit' is valid for C++/ObjC++ but not for C [-Werror]

But, overriding the weak symbol __register_exitproc() seems to work, saving almost 300 bytes:

Before: 

   text    data     bss     dec     hex filename
  14296     188    3576   18060    468c cannette_fw 

With override:

   text    data     bss     dec     hex filename
  13912     184    3432   17528    4478 cannette_fw 

[fenugrec]

closed via #147