meson: pg_regress: Define a HOST_TUPLE sufficient to make resultmap work
This doesn't end up with a triple that's exactly the same as config.guess -
it'd be hard to achieve that and it doesn't seem required. We can't rely on
config.guess as we don't necessarily have a /bin/sh on windows, e.g., when
building on windows with msvc.
This isn't perfect, e.g., clang works on windows as well. But I suspect we'd
need a bunch of other changes to make clang on windows work, and we haven't
supported it historically.
Discussion: http://postgr.es/m/
20220928022724.erzuk5v4ai4b53do@awork3.anarazel.de