[Relax][TVMScript] Print ExternFunc struct_info when non-default (#19416)

### Summary

1. Add HasDefaultExternFuncStructInfo helper to detect default
FuncStructInfo for extern functions.

2. Update relax::ExternFunc printer to:
 - emit global_symbol using the correct AccessPath attribute key,
- conditionally include struct_info only when it differs from the
default inferred-by-sinfo-args derive function,
- use a variadic args array instead of a single positional literal to
prepare the ExternFunc call.

3. This reduces noisy/redundant output when printing ExternFunc nodes
while preserving explicit struct_info when it conveys meaningful
information.

---------

Co-authored-by: cchung100m <cchung100m@users.noreply.github.com>