The Xen headers support only x86 and ARM. Allow for platforms such as
LoongArch64 to build despite the absence of Xen support by providing
an architecture-specific <bits/xen.h> that simply does:
#ifndef _BITS_XEN_H
#define _BITS_XEN_H
#include <ipxe/nonxen.h>
#endif /* _BITS_XEN_H */
Signed-off-by: Michael Brown <mcb30@ipxe.org>