cranelift-object: Report error when symbol declared as local or exported is not defined.

[bjorn3]

Feature

See title.

Benefit

This makes debugging this kind of issue easier by reporting an error when writing the object file rather than feeding the linker a corrupt object file. This also allows finding the name of the exact symbol that is at fault.

Implementation

This check can be done in the finish method.

Alternatives

TODO: What are the alternative implementation approaches or alternative ways to
solve the problem that this feature would solve? How do these alternatives
compare to this proposal?