Further tidy-up for old CPU architectures.

Further tidy-up for old CPU architectures.

Further to commit 92d70b77, let's drop the code we carry for the
following untested architectures: M68K, M88K, M32R, SuperH.  We have no
idea if anything actually works there, and surely as vintage hardware
and microcontrollers they would be underpowered for modern purposes.

We could always consider re-adding SuperH based on evidence of usage and
build farm support, if someone shows up to provide it.

While here, SPARC is usually written in all caps.

Suggested-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Robert Haas <robertmhaas@gmail.com> (the idea, not the patch)
Discussion: https://postgr.es/m/959917.1657522169%40sss.pgh.pa.us