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 script doesn't know about the
exe wrapper and hence fails miserably. I discussed this a bit with
Dylan and he filed an upstream meson issue:

https://github.com/mesonbuild/meson/issues/4427#issuecomment-433553689



Another option would be to use binfmt-misc handling (which works
neatly locally), but on the runners we have I can't control that.
Maybe I can beg Daniel Stone ...

For not let's just mark this stage as optional and that failures are
allowed.

v2: Use the new gitlab 1.4 feature to rebuild docker images when
necessary.

Cc: Sean Paul <sean@poorly.run>
Cc: Eric Anholt <eric@anholt.net>
Cc: Dylan Baker <dylan@pnwbakers.com>
Cc: Daniel Stone <daniel@fooishbar.org>
Reviewed-by: Eric Anholt <eric@anholt.net> (v1)
Reviewed-by: Petri Latvala <petri.latvala@intel.com> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>