gdb: make so_list::{so_original_name,so_name} std::strings

Change these two fields, simplifying memory management and copying.

Change-Id: If2559284c515721e71e1ef56ada8b64667eebe55
Approved-By: Pedro Alves <pedro@palves.net>
Reviewed-By: Reviewed-By: Lancelot Six <lancelot.six@amd.com>