compile/runtime testing for __cxa_atexit

[Benjamin Kosnik]

> your plan makes perfect sense to me. 

Ok. It's actually easy to do, just ripping off your namedlocale work
with a different test case, variables, functions, etc.

> Do you want me to work on this issue or maybe you
> are going to take care of it together with the mt_allocator testing
> work? Just let me know...

I got it. I was just curious, because this seems pretty basic and I was
surprised that there wasn't already infrastructure for testing this.

-benjamin