A few highlights relative to our existing CI:
- Add a FreeBSD 12.2 and 13.0 runner so we have some FreeBSD CI.
- Windows builds use msbuild with the Visual-Studio-provided clang toolchain to test clang
- The matrix builds describe the axes of the matrix, not all points.
- The Arm builds now cross-compile with a native clang and run the tests with qemu, rather than running the compiler, linker, and ctest all with qemu.
This also includes a fix for one of the tests that was doing `static_cast<unsigned int>(1) << 36`, which is undefined behaviour and was sometimes causing qemu to hang. There is now an assert to catch this in the future.
