[Bug web/85578] broken links in gcc-8.0.1-RC-20180427/INSTALL/specific.html, and out of date prerequisites.html

[andrewm.roberts at sky dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85578

--- Comment #2 from Andrew Roberts <andrewm.roberts at sky dot com> ---
Ok thanks, just checking on the prerequisites front.