rump kernels: use new platform macro

rump kernels: use new platform macro

Starting from rump kernel changeset 91d5623 ("Renaming platform macros,
app-tools and autoconf target string"), __RUMPUSER_XEN__ and __RUMPAPP__
are deleted. We are supposed to use __RUMPRUN__ instead.

We still keep __RUMPUSER_XEN__ for now in order to make xen-unstable
pass osstest push gate. I will remove __RUMPUSER_XEN__ later.

Related discussion:
http://thread.gmane.org/gmane.comp.rumpkernel.user/739

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>