[Bug bootstrap/67728] Build fails when cross-compiling with in-tree GMP and ISL

["bneumeier at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67728

--- Comment #6 from Brett Neumeier <bneumeier at gmail dot com> ---
Created attachment 36441
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36441&action=edit
document symptoms of problem and workaround

Since the issue is a corner-case (it's only triggered when cross-compiling for
certain architectures, using recent versions of GMP, and with in-tree GMP and
ISL sources), and since there's an easy and obvious workaround, it seems
pointless to spend a lot of time trying to adjust the build machinery.

On the other hand, a something search reveals I'm not the only person to run
into this situation, so it seems worthwhile to at least document the
workaround. Here's a suggested paragraph -- it would have saved *me* a headache
and a few hours of frustration when I tripped across the problem, so perhaps it
will be helpful to someone else in the future?