A patch for visibility.

[Martin von Loewis]

> I believe we should clear the visibility field for undefined symbols.
> What do you think?

No, I believe the intent is that visibility should work for undefined
symbols, as well. It is an error if a hidden or private symbols is not
present when linking a shared executable. I don't know what should
happen for a undefined protected symbol - it would probably be ok to
clear the visibilityl, but it should do no harm to keep it.

Besides: there are still some systems that use st_other in an other
meaning. It would probably be good if we treat those gracefully - I
tried the best I could.

Regards,
Martin