Replace -Z ast-json in the check for undefined symbols

[tschorr]

-Z ast-json ist no longer available in Rust nightly, see also #279 (comment). Suggestion: Let nm report undefined symbols. This should also work for Rust stable.

[tschorr]

Could someone approve running the workflow? This will most likely need some tweaks.

[dgrunwald]

I think nm will only report functions that are actually called, I wanted to check all functions that are declared even if unused.

[tschorr]

all functions that are declared even if unused.

I'm not exactly sure what that should cover and I don't have the -Z ast-json output to compare the results. Could you elaborate?
nm -u will report undefined symbols (those external to each object file). It seems to make sense to check if those are provided in the so_files. But there is also nm --defined-only that will report symbols in the text and data sections.

There is also the nightly -Z unpretty=ast-tree,expanded flag to rustc that might provide what you're looking for. But the output ist different from -Z ast-json (e.g. it's not json).

[tschorr]

After looking a bit more into the output of the different commands, I have decided to go with -Z unpretty=ast-tree,expanded as it seems closer to the original test than the other options. Also, the .rlib contents differ a lot depending on the link mode.