Better explain why there's multiple GCCs built [trac #1119]

[bradfa]

Asked by Chris Smith on the support mailing list: ​http://lists.clfs.org/pipermail/clfs-support-clfs.org/2016-September/002077.html

We don't make it entirely clear why you have to build GCC twice. Clear that up.

This is trac ticket: https://trac.clfs.org/ticket/1119