gitlab-ci: Switch to clang-tools

clang-3.9 does not contain scan-build anymore.
4 jobs for fedora in 2 minutes and 57 seconds
Status Name Job ID Coverage
  Build
passed build-debian-clang #4579
librem5

00:02:36

passed build-debian-gcc #4578
librem5

00:02:11

25.4%
passed build-debian-package #4581
librem5

00:02:16

passed build-fedora-gcc #4580
librem5

00:02:56