Make x86-lynx GDBserver pass XML register map to GDB.

Make x86-lynx GDBserver pass XML register map to GDB.

This is not strictly needed, since both GDB and GDBserver seem
to agree on the register numbering without this.  But this allows
us to make sure that this is always going to be the case.

gdb/gdbserver/ChangeLog:

        * configure.srv (i[34567]86-*-lynxos*): Set srv_xmlfiles.