GNU indirect functions vs. symbol visibility

[Florian Weimer]

* Alan Modra:

> glibc people: As the main user of ifuncs, how do you feel about not
> declaring functions hidden that are implemented in glibc by ifuncs?

We have run into this before, I think:

  <https://sourceware.org/ml/libc-alpha/2016-07/msg00089.html>

> It's fine to make them hidden via a version script, or even define
> them as hidden (which requires just the rs6000_elf_encode_section_info
> part of my gcc patch to make ppc32 behave).

If it doesn't work, we'd certainly prefer an early diagnostic.