Do a better job of filtering Mach-O symbol tables

[woodruffw]

Symbol tables in Mach-Os can contain all kinds of junk, including symbolic debug entries and entries without actual names. This can result in a lot of unnecessary work, since we end up scanning symbols that don't actually correspond to the CPython ABI being linked against.

The solution here is to filter the symbol table, and only audit symbols that correspond to function or data entries and that are marked as "undefined" (meaning external, not local).

[woodruffw]

Thinking about this more, I'm not as confident this is correct: we want to catch CPython extensions that have non-abi3 APIs inlined into them, and filtering non-external functions will prematurely exclude those.

The more correct thing to do here is to filter out anything that doesn't resemble a function or data object, not all non-externals.