xen/Makefile: remove all temporary files for every architecture

xen/Makefile: remove all temporary files for every architecture

Execute the clean target for both arm and x86 architecture.

When trying to build Xen for a different architecture in the same
tree, the command make clean will only remove temporary files for
the host architecture.
This will lead a compilation error when trying to build ARM64 and
ARM32 Xen in the same tree.
(See also: https://lists.xenproject.org/archives/html/xen-devel/2016-11/msg02176.html)

Signed-off-by: Luca Miccio <lucmiccio@gmail.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>