Fix libc-abis handling for add-ons

[Roland McGrath]

Ports add-ons should not be overriding main files like that.
The intent of the add-ons system for ports is that you should
be able to enable any and all ports add-ons and they have no
effect unless you are building for an affected configuration.

I've instead changed the handling of the libc-abis file to match that of
shlib-versions files.  That means you can have them in sysdeps or add-on
subdirectories, and all found are concatenated together.  Hence the
contents of an add-on file should not replicate the architecture-independent
lines in the main file (doing so will screw up the calculation).


Thanks,
Roland