gdb: replace some so_list parameters to use references

gdb: replace some so_list parameters to use references

A subsequent patch changes so_list to be linked using
intrusive_list.  Iterating an intrusive_list yields some references to
the list elements.  Convert some functions accepting so_list objects to
take references, to make things easier and more natural.  Add const
where possible and convenient.

Change-Id: Id5ab5339c3eb6432e809ad14782952d6a45806f3
Approved-By: Pedro Alves <pedro@palves.net>
Reviewed-By: Reviewed-By: Lancelot Six <lancelot.six@amd.com>