Remove support for HPPA (a/k/a PA-RISC) architecture.

Remove support for HPPA (a/k/a PA-RISC) architecture.

This old CPU architecture hasn't been produced in decades, and
whatever instances might still survive are surely too underpowered
for anyone to consider running Postgres on in production.  We'd
nonetheless continued to carry code support for it (largely at my
insistence), because its unique implementation of spinlocks seemed
like a good edge case for our spinlock infrastructure.  However,
our last buildfarm animal of this type was retired last year, and
it seems quite unlikely that another will emerge.  Without the ability
to run tests, the argument that this is useful test code fails to
hold water.  Furthermore, carrying code support for an untestable
architecture has costs not to be ignored.  So, remove HPPA-specific
code, in the same vein as commits 718aa43a4 and 92d70b77e.

Discussion: https://postgr.es/m/3351991.1697728588@sss.pgh.pa.us