gdb: Workaround bad gdbserver qSupported:xmlRegisters=i386;UnknwnFeat+ handling

gdbserver's target_process_qsupported is called for each feature that
the gdbserver common code does not recognize.  The only current
implementation, for x86 Linux, does this:

  static void
  x86_linux_process_qsupported (const char *query)
  {
    /* Return if gdb doesn't support XML.  If gdb sends "xmlRegisters="
       with "i386" in qSupported query, it supports x86 XML target
       descriptions.  */
    use_xml = 0;
    if (query != NULL && startswith (query, "xmlRegisters="))
      {
	char *copy = xstrdup (query + 13);
	char *p;

	for (p = strtok (copy, ","); p != NULL; p = strtok (NULL, ","))
	  {
	    if (strcmp (p, "i386") == 0)
	      {
		use_xml = 1;
		break;
	      }
	  }

	free (copy);
      }

    x86_linux_update_xmltarget ();
  }

Notice that this clears use_xml and calls x86_linux_update_xmltarget
each time target_process_qsupported is called.  So if gdb sends in any
unknown feature after "xmlRegisters=i386", like e.g.,
"xmlRegisters=i386;UnknownFeature+" gdbserver ends up not reporting a
XML description...

Work around this by having GDB send the "xmlRegisters=" feature last.

gdb/ChangeLog:
2015-11-19  Pedro Alves  <palves@redhat.com>

	* remote.c (remote_query_supported): Send the "xmlRegisters="
	feature last.