xserver_xorg-server: add support for xtensa architecture

Fixes:
  http://autobuild.buildroot.net/results/143/14308a9d1806574f06ab2a7d222f53119fab1c90/
  http://autobuild.buildroot.net/results/5cc/5cceb96ac29a410316fd7d8f7de0615ba33e3f2c/



Backported from: dc8d0688471695ec5a8db5fef93fbcfc064891d5
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>