automation: add clang and lld 8 tests to gitlab

automation: add clang and lld 8 tests to gitlab

Using clang and lld 8 requires installing the packages from the
official llvm apt repositories, so modify the Debian Docker files for
stretch and unstable to add the llvm repo and install clang and lld
from it.

Also add some jobs to test building Xen with clang 8 and lld.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Doug Goldstein <cardoe@cardoe.com>