Allow powerpc-*-lynxos* GDBserver to send register map via XML...

Allow powerpc-*-lynxos* GDBserver to send register map via XML...

... back to GDB.  The transfer occurs when GDB sends the
'qXfer:features:read:target.xml' packet.  This allows us to make
sure that GDB and GDBserver use the same register numbering.
This is important on Lynx 178, where GDB selects the rs6000:6000
architecture by default instead of the powerpc:common architecture.

gdb/gdbserver/ChangeLog:

        * configure.srv (powerpc-*-lynxos*): Set srv_xmlfiles.