natdynlink (and ocamlnat) should be disabled with --disable-shared

[dra27]

On platforms which support it, dynlink.cmxa is still built even when OCaml is configured with --disable-shared. This doesn't cause build issues, but given that it fundamentally doesn't work we should disable dynlink.cmxa when configured with --disable-shared.

The same is not true for dynlink.cma, even for code which uses C stubs, because you could compile the bytecode on a system without shared library support and then run it on a system where ocamlrun does have shared library support.

Spotted while dealing with #10690; we'll see whether I get to this before the stale bot nags me.

[github-actions]

This issue has been open one year with no activity. Consequently, it is being marked with the "stale" label. What this means is that the issue will be automatically closed in 30 days unless more comments are added or the "stale" label is removed. Comments that provide new information on the issue are especially welcome: is it still reproducible? did it appear in other contexts? how critical is it? etc.