xserver_xorg-server: add support of ARC architecture

Support of ARC was added after 1.15.1 release so until the next official release
happens this patch enables building and execution of Xorg server on ARC.

Upstream commit:
http://cgit.freedesktop.org/xorg/xserver/commit/?id=c08d2b8ccb0158dbf1f529f80cc3236e66236cce



Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>

Cc: Bernd Kuhls <berndkuhls@hotmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>