tests/docker: use debian-all-test-cross for m68k

Maintaining two sets of containers for test building is silly. While
it makes sense for the QEMU cross-compile targets to have their own
fat containers built by lcitool we might as well merge the other
random debian based compilers into the same one used on gitlab.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231029145033.592566-11-alex.bennee@linaro.org>

configure

@@ -1313,6 +1313,11 @@ probe_target_compiler() {
         container_image=debian-loongarch-cross
         container_cross_prefix=loongarch64-unknown-linux-gnu-
         ;;
+      m68k)
+        container_image=debian-all-test-cross
+        container_cross_prefix=m68k-linux-gnu-
+        container_cross_cc=${container_cross_prefix}gcc
+        ;;
       microblaze)
         container_image=debian-microblaze-cross
         container_cross_prefix=microblaze-linux-musl-