CI: Add Clang build

Add a build with clang and lld to get more test coverage. There are a couple of trivial type-mismatch fixes in here, as well as shifting PipeWire to cherry-pick pipewire/pipewire!858 (merged), and a very unpleasant change to the test suite to arbitrarily bump structure alignment to accord better with how clang/lld want to lay out our structures in the dedicated section. I would rather do anything else than the latter, but I would also really rather not rewrite our test suite again, and ... this works.