ci: Build stage4 and run behavior tests with it on aarch64-linux-debug.

This ensures that we at least have some relatively lightweight coverage of
building and using stage4 from a newly built stage3.

ci/aarch64-linux-debug.sh

@@ -56,3 +56,13 @@ stage3-debug/bin/zig build test docs \
   --search-prefix "$PREFIX" \
   --zig-lib-dir "$PWD/../lib" \
   -Denable-superhtml
+
+stage3-debug/bin/zig build \
+  --prefix stage4-debug \
+  -Denable-llvm \
+  -Dno-lib \
+  -Dtarget=$TARGET \
+  -Duse-zig-libcxx \
+  -Dversion-string="$(stage3-debug/bin/zig version)"
+
+stage4-debug/bin/zig test ../test/behavior.zig