Xfer:features:read truncating xml sent to gdb frontends

Around line 1326 in gdbstub.c:

            if (len > (MAX_PACKET_LENGTH - 5) / 2)
                len = (MAX_PACKET_LENGTH - 5) / 2;

is truncating processor reg description xml files longer than 2045 bytes.  Deleting these lines works for my immediate need, but they seem to be trying to fix some buffer overrun condition so I won't offer a patch until we understand their purpose.