Document use of IFUNC support outside of libc.

[Carlos O'Donell]

Szabolcs,

I attempted to distill some of your notes here:
https://sourceware.org/glibc/wiki/GNU_IFUNC

That way I can point users at this.

In gperftools tcmalloc added an IFUNC use [1] which
violates some of the requirements under -Wl,z,now,
so I have a need to document this support and discuss
with tcmalloc developers what we might do. Right now
they call way too much code for this to work.

Cheers,
Carlos.

[1] https://github.com/gperftools/gperftools/commit/6fdfc5a7f40ebcff3fdaada1a2994ff54be2f9c7