gitlab-ci: Switch to clang-tools

clang-3.9 does not contain scan-build anymore.
4 jobs for master in 2 minutes and 51 seconds
Status Job ID Name Coverage
  Build
passed #4583
librem5
build-debian-clang

00:02:37

passed #4582
librem5
build-debian-gcc

00:02:12

25.4%
passed #4585
librem5
build-debian-package

00:02:15

passed #4584
librem5
build-fedora-gcc

00:02:51