core_xfer_shared_libraries and core_xfer_shared_libraries_aix returns ULONGEST
This patch documents the return value of core_xfer_shared_libraries_aix
and core_xfer_shared_libraries gdbarch methods, and changes return
type to ULONGEST from LONGEST.
In a following patch, core_xfer_partial. is changed to check their
return values and return an appropriate target_xfer_status.