Remove the old table of "supported platforms" in favor of a link to the

Remove the old table of "supported platforms" in favor of a link to the
buildfarm plus a narrative description of the CPU types and operating systems
on which Postgres is likely to work.  Now that we've almost completely
decoupled CPU and OS considerations, the former tabular style isn't all that
enlightening anyway.  Perhaps more importantly, no one seems particularly
interested in maintaining the table by hand when we have the buildfarm.