We are doing a lot of cross-compilation tests with GCC in the gitlab-CI
already, so we could get some more test coverage by using Clang in the
Travis-CI instead. Thus let's switch two additional jobs to use Clang
for compilation.

Message-ID: <20240320104144.823425-7-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>