Take into account all flags when computing iface_hash

The "interface hash" should contain a hash of everything in the
interface file. We are not doing that yet but at least a start is to
include a hash of everything in `mi_self_recomp`, rather than just
`mi_src_hash` and `mi_usages`.

In particular, this fixes #25837, a bug where we should recompile a
`dyn_hi` file but fail to do so.