Adjust the GitLab CI jobs to match the new images

Adjust the GitLab CI jobs to match the new images

The custom builds (oot, asan, tsan) were mostly built using Debian sid
amd64 image.  The problem was that this image broke too easily, because
it's Debian "unstable" after all.

This commit introduces "base_image" that should be most stable with
extra bits on top (clang, coccinelle, cppcheck, ...).  Currently, that
would be Debian buster amd64.

Other changes introduced by this commit:

* Change the default clang version to 10
* Run both ASAN and TSAN with both gcc and clang compilers
* Remove Clang Debian stretch i386 job