xen: arm: refactor 64-bit return from trap path

xen: arm: refactor 64-bit return from trap path

Refactor exit path to use a single "exit" macro similar to the entry path.

We can also remove the logic at "return_to_new_vcpu" which detects returns to
hypervisor mode -- seemingly trying to handle hypervisor threads which aren't
an thing which we have. The idle VCPUs do not take this path. This simplifies
the return_to_new_vcpu code, we also split it into 32- and 64-bit VCPU paths.

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