Work around Mesa issues by ignoring test failures on !amd64

LLVM support in Mesa also implies linking to the GPL-2/LGPL-3 libelf.

On amd64 we can simply move the LLVM drivers to their own package, ensuring that it only gets pulled in at build time and on the SDK images.

On other platforms at the moment we have to choose:

1. disable LLVM, losing GL support via llvmpipe in swrast, breaking the GL tests under Xvfb
2. enable LLVM, with the the etnaviv driver linking to libelf via gallium

Since option 2 is not unacceptable for products, let's simply disable the Xvfb tests everywhere except on amd64 until we find a better fix.

See https://phabricator.apertis.org/T6447 for more details.

See also this test build on OBS succeeding.