Replace `omega` tactic with `lia`
Since Coq 8.12, `omega` is flagged as deprecated and scheduled for removal. Also replace CompCert's homemade tactics `omegaContradiction`, `xomega`, and `xomegaContradiction` with `lia` and `extlia`. Turn back on the deprecation warning for uses of `omega`. Make the proof of `Ctypes.sizeof_pos` more robust to variations in `lia`.