Remove leftovers of IRIX port

This removes the remaining pieces of the IRIX port that was removed by
ea91a6be89575095f61ebf36d67c2df98be093db.
Fix doc links in README file to work with new website layout

Per report from Colin 't Hart
doc: Fix typo.

Reported by Thom Brown.
Try again to make pg_isolation_regress work its build directory.

We can't search for the isolationtester binary until after we've set
up the environment, because otherwise when find_other_exec() tries
to invoke it with the -V option, it might fail for inability to
locate a working libpq.  So postpone that step.

Andres Freund