ARM: Xen: fix initial build problems

* The XEN_BALLOON code requires the balloon infrastructure that is not
  getting built on ARM.

* The tmem hypercall is not available on ARM

* ARMv6 does not support cmpxchg on 16-bit words that are used in the
  Xen grant table code, so we must ensure that Xen support is only
  built on ARMv7-only kernels not combined ARMv6/v7 kernels.

* sys-hypervisor.c needs to include linux/err.h in order to use the
  IS_ERR/PTR_ERR/ERR_PTR family of functions.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: xen-devel@lists.xensource.com