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

drivers/xen/Kconfig

@@ -3,6 +3,7 @@ menu "Xen driver support"
 
 config XEN_BALLOON
 	bool "Xen memory balloon driver"
+	depends on !ARM
 	default y
 	help
 	  The balloon driver allows the Xen domain to request more memory from
@@ -145,6 +146,7 @@ config SWIOTLB_XEN
 
 config XEN_TMEM
 	bool
+	depends on !ARM
 	default y if (CLEANCACHE || FRONTSWAP)
 	help
 	  Shim to interface in-kernel Transcendent Memory hooks