The physmem KPI is already used on arm, arm64, and riscv64; extend it
towards x86. This reduces some complexity in machdep.c in favor of
using platform-common coalescing logic and being a bit easier to grok
at a glance.
ah, remnants of a former versiin... this used to just assert on the exclude return rather than bail out. Will remove.
This one is a brain-o. We do want to stop, but only if we hit an E2BIG (or whatever the "region full" error ends up being), since there's no point in testing once we can't exclude anything else. I think the previous version emitted a notice along those lines, so I'll restore that and fix this.