Archive Pull-In Semantics

Implement int archive_pulls(size_t used_syms, size_t defined_syms, int member_needed) modeling a linker decision: given an archive member defining defined_syms symbols of which the link currently needs used_syms (0 if none), and member_needed 1 iff the member defines a currently-undefined referenced symbol — return 1 iff the member gets pulled into the link. This models: pull-in happens exactly when the member resolves a pending undefined reference.