pl/math: Auto-generate mathbench and ulp headers

Instead of maintaining three separate lists of routines, which
are cumbersome and prone to merge conflicts, we provide a new
macro, PL_SIG, which by some preprocessor machinery outputs the
lists in the required format (macro formats have been changed
very slightly to make the generation simpler). Only routines with
simple signatures are handled - binary functions still need
mathbench wrappers defined manually. As well, routines with
non-standard references (i.e. powi/powk) still need entries and
wrappers manually defined.