gitlab-ci: Expand CI job for building with a C++ compiler

This tests out if libweston can be built with a C++ compiler and catch
potential issues which we don't normally see.

We re-use one of the builds, the full-x86-64 one, and build this new
front-end alongside it.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>