dockerfiles: fix default targets for debian-loongarch-cross

dockerfiles: fix default targets for debian-loongarch-cross

fix system target name, and remove --disable-system (which deactivates
system target).

Found using: make docker-test-build@debian-loongarch-cross V=1

Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20241020213759.2168248-1-pierrick.bouvier@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20241023113406.1284676-10-alex.bennee@linaro.org>
(cherry picked from commit 24be5341fbeea341cca38b59d4c0928a8cf5fac1)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>