Create a hook for inspecting program headers during library load

[Matthew Fortune]

> On 2 October 2014 16:02, Matthew Fortune <Matthew.Fortune@imgtec.com>
> wrote:
> > Patch updated below. Though I did remember Joseph steering me away from
> > an ifdef based interface while reworking this:
> >
> > https://sourceware.org/ml/libc-alpha/2014-05/msg00045.html
> >
> > Does this seem more appropriate?
> 
> You could implement it using the sysdeps mechanism, e.g. see how
> dl-irel.h and similar headers operate.

Thanks. I didn't quite follow what Roland was referring to by a sysdeps file.
That will be much neater.

Matthew