xen: arm: correct platform detection in public header.

xen: arm: correct platform detection in public header.

These headers cannot use the CONFIG_FOO defines provided when building Xen
(since they aren't provided when building tools or by external components) and
need to use the compiler provided architecture defines.

This manifested itself as a failure to build xenctx.c on ARM64 due to the
missing symbols contains .

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>