[lld][WebAssembly] Fix use of undefined funcs under --warn-unresolved-symbols (#78643)

When undefined functions exist in the final link we need to create
stub functions (otherwise direct calls to those functions could
not be generated).  We were creating those stub when
`--unresolved-symbols=ignore-all` was passed but overlooked the fact
that `--warn-unresolved-symbols` essentially has the same effect (i.e.
undefined function can exist in the final link).

Fixes: #53987