gitlab-ci.d/buildtest: Rework the target list of build-system-alpine

gitlab-ci.d/buildtest: Rework the target list of build-system-alpine

The target list of the build-system-alpine job is pretty much a copy
of the build-system-ubuntu job (apart from "aarch64-softmmu" which
has recently been removed from the ubuntu job in commit 6eda5ef5f8f4,
but aarch64-softmmu is still also tested in the opensuse jobs, so
we don't need to keep it here).

Let's stop wasting our CI minutes with such duplications, and focus
on testing targets instead that do not have such a great test coverage
yet: The "loongarch64-softmmu" target has never been added to our
build tests yet since it has been introduced, and the "mips64-softmmu"
target is so far only tested in jobs that lack the "avocado" testing
stage (only the little endian or 32-bit MIPS variants are tested in
jobs with avocado so far).

While we're at it, also move the avr-softmmu and mipsel-softmmu targets
from the Debian job to the alpine job, since the Debian job (and its
following test jobs) has already a long runtime compared to the others
jobs. With this movement, the runtimes should be more equally distributed
along the parallel running jobs now.

Message-Id: <20230309164850.109882-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>