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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/718aa43a4ee613bf85afadb1596f861e1228ca43

Modified Files
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doc/src/sgml/installation.sgml | 5 +-
src/backend/storage/lmgr/s_lock.c | 65 ------------------------
src/include/storage/s_lock.h | 102 --------------------------------------
3 files changed, 2 insertions(+), 170 deletions(-)