gatomic: Cast the oldval pointer type to the atomic value type, and add msys2-clang64 scheduled CI job
If glib_typeof is defined we can try to cast the old value to the atomic value type to check if it's compatible with the pointer one.
Closes: #2798 (closed) (see https://gitlab.gnome.org/3v1n0/glib/-/jobs/2361884)