xen: arm: use new 64-bit zImage magic numbers for Xen binary

xen: arm: use new 64-bit zImage magic numbers for Xen binary

Upstream commit 4370eec05a88 "arm64: Expand arm64 image header" ended up
changing the zImage magic (which was actually the initial branch instructio
encoding!). The new header has a proper magic number at a fixed location.

Switch Xen itself to using this format. Neither the bootwrapper nor the
models care about this header themselves and real bootloaders are not widely
used, so now is as good a time as any to switch (as upstream have proven)

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>