Add ARM support into xf86drm.h

This provides support for Xorg interface.  Without this the vivante
samples will hang during close requiring a reboot

[Adapted from yocto project]
Upstream-Status: Pending
Signed-off-by: Lauren Post <lauren.post@freescale.com>
Signed-off-by: Evan Kotara <evan.kotara@freescale.com>
[Thomas: change CAS code to only be used on ARMv6/ARMv7, and not
ARMv4/ARMv5, which don't support ldrex/strex. If no CAS implementation
is provided libdrm falls back to a system call for locking/unlocking.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Acked-by: Eric Engestrom <eric.engestrom@intel.com>
---
Changes v1 -> v2:
  - add comment explaining exclusion of ARMv4/ARMv5 and lower