Move arm from ports to libc

[Roland McGrath]

> The aim is that the ports directory ends up containing only the old 
> ChangeLog files from the time ports were kept there, and nothing else 
> (well, maybe a minimal README).  See bullet point 6 in 
> <https://sourceware.org/ml/libc-alpha/2014-01/msg00373.html>.

In the long run, I think the existence of the directory will just be
confusing.  When everything is moved, all the ChangeLog.foo files can move
to the top.