gdb: make solib-rocm not use so_list internally

Same rationale as the previous patch, but for solib-rocm.

 - Introduce rocm_so, which is a name a unique_name (see comment in
   rocm_update_solib_list for that) and a unique_ptr to the
   lm_info_svr4.
 - Change the internal lists from so_list lists to vectors of rocm_so.
 - Remove rocm_free_solib_list, as everything is automatic now.
 - Replace rocm_solib_copy_list with so_list_from_rocm_sos.

Change-Id: I71e06e3ea22d6420c9e4e500501c06e9a13398a8
Approved-By: Pedro Alves <pedro@palves.net>
Reviewed-By: Reviewed-By: Lancelot Six <lancelot.six@amd.com>