[v3] remove cpu_limits.h support

[Benjamin Kosnik]

> > Can't that be done in os_defines.h?
> 
> Um, sure, I guess.

Great.

I'll give Gaby a chance to OK, and then check it in. Since none of the
current targets need this, and theoretically os_defines.h can handle it
if it ever becomes necessary, and cpu_limits.h was a bit dodgy in the
first place, I think this might be a fine time to part ways with the
file.

-benjamin