Add generic ARGn register IDs for FreeBSD and Linux x86

These were previously missing in g_register_infos_x86_64 and so arg
lookup failed on FreeBSD and Linux after r194035.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@194392 91177308-0d34-0410-b5e6-96231b3b80d8