gdb: use gdb::checked_static_cast when casting lm_info

Now that the lm_info class hierarchy has a virtual destructor and
therefore a vtable, use checked_static_cast instead of C-style cases to
ensure (when building in dev mode) that we're casting to the right kind
of lm_info.

Change-Id: I9a99b7d6aa9a44edbe76377d57a7008cfb75a744
Approved-By: Pedro Alves <pedro@palves.net>
Reviewed-By: Reviewed-By: Lancelot Six <lancelot.six@amd.com>