Fix libc-abis handling for add-ons

[Roland McGrath]

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> On 04/07/2010 10:49 AM, Roland McGrath wrote:
> > No, it wasn't.  As I've already said, it breaks the fundamental rules of
> > how ports add-ons must be.  It's possible we need to make things slightly
> > more complicated to do it right.  But this is not an acceptable solution.
> 
> Of course it is acceptable.    The problem with any of these ABI
> definitions is to not allow it to be screwed up by an incorrect change.
>  This is why I decided on the rules "always add at the end".  With this
> each feature has a constant ordering.

You misunderstood me.  I'm not talking about the contents of the file.
I'm talking about the method of integrating add-ons.