AIX: Consider tdata symbols in ExportImportList

Thread-local variables are put in the tdata section on AIX / XCOFF [1]. Unfortunately
the ExportImportList script currently doesn't consider this section when looking for
symbols to export.

This results in linking failures in applications (such as LLVM) when building which expect these thread local variables to be exported from their shared libraries.

[1]: https://www.ibm.com/docs/en/aix/7.3?topic=formats-xcoff-object-file-format