Tidy up claimed supported CPUs and OSes.

Tidy up claimed supported CPUs and OSes.

 * Remove arbitrary mention of certain endianness and bitness variants;
   it's enough to say that applicable variants are expected to work.
 * List RISC-V (known to work, being tested).
 * List SuperH and M88K (code exists, unknown status, like M68K).
 * De-list VAX and remove code (known not to work).
 * Remove stray trace of Alpha (support was removed years ago).
 * List illumos, DragonFlyBSD (known to work, being tested).
 * No need to single Windows out by listing a specific version, when we
   don't do that for other OSes; it's enough to say that we support
   current versions of the listed OSes (when 16 ships, that'll be
   Windows 10+).

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Greg Stark <stark@mit.edu>
Discussion: https://postgr.es/m/CA%2BhUKGKk7NZO1UnJM0PyixcZPpCGqjBXW_0bzFZpJBGAf84XKg%40mail.gmail.com