[uClibc] __cxa_atexit/__cxa_finalize complete implementation

[Joakim Tjernlund]

> 
> Possible issues:
> 
> * I included <bits/atomicity.h> to implement __cxa_finalize (I need
> compare_and_swap). When I do a non-debug build of uClibc, this file
> generates warnings saying "warning: matching constraint does
> not allow a register". When I build a debug uClibc, this file generates
> an errot!

Looks like stuff in atomicity.h is broken. Futhermore from what I can gather from the www,
atomicity.h should die and be relplaced by atomic.h. Probably best to copy that
stuff from glibc.

 Jocke