ci: Add 32bit build and test
It's easy enough to make changes that break on 32bit platforms and few people
test that locally. Add a test for that to CI. LLVM is disabled on 32bit
because installing it would bloat the image size further.
The debian w/ meson task is fast enough that we can afford to test both.
Use the occasion of a separate run of the tests to run them under LANG=C,
we've recently discovered there's not a lot of testing in the buildfarm for
the case.
Discussion: https://postgr.es/m/
4033181.
1664395633@sss.pgh.pa.us