gdb/dwarf: make all fields of cutu_reader private

Make all fields of cutu_reader private, then add getters for whatever
needs to be accessed outside of cutu_reader.  This should help spot
what's used by cutu_reader itself, and what is used by others.

Change-Id: I71cb73fffa5d70cc9c7fc68bf74db937e84c2db1
Approved-By: Tom Tromey <tom@tromey.com>