gitlab-ci: arm64 cross compiling

First I tried to build in a full sysroot, but I didn't figure out why
ld never found the libraries (this was on fedora, which might have
been part of the problem - if you just need a toolchain for kernel
corss-building fedora is perfectly fine).

Then I tried using debian's multiarch stuff, which worked a lot
better, except there's some minor gotchas:
- some libraries conflict and can't be multi-arch installed
- some libraries are buggy and cause conflicts when installing
- pkgconfig and ld get confused with some other libraries and pick the
  first one they find (i.e. the x86_64 one, not the arm64 one).
That means we need a new docker image with just the cross libaries
installed as -dev packages, and none of the native ones.

Next up I tried to run the tests. Meson has really nice support for an
exe wrapper, and qemu happily runs arm64 on x86_64. With that I
managed to run some tests successfully (it's not even that slow!). But
most are run through a script, and the scrip...