gitlab-ci: Install most packages from Debian buster

We now use the C frontend of GCC 8 instead of 6 (required tweaking the
before_script for the clang job). We cannot use the C++ frontend of GCC
7 or newer yet, because upstream GCC 7 changed some C++ name mangling
stuff in backwards incompatible ways, and LLVM < 6.0 packages aren't
available in buster.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
2 files changed