gdb/solib-svr4: add get_lm_info_svr4

Add this function, as a shortcut of doing the more verbose:

    auto *li = gdb::checked_static_cast<lm_info_svr4 &> (*solib.lm_info);

Change-Id: I0206b3a8b457bdb276f26b354115e8f44416dfcf
Reviewed-By: Guinevere Larsen <guinevere@redhat.com>