xen/tasklet: Fix return value truncation on arm64

xen/tasklet: Fix return value truncation on arm64

The use of return_reg() assumes ARM's 32bit ABI.  Therefore, a failure such as
-EINVAL will appear as a large positive number near 4 billion to a 64bit ARM
guest which happens to use continue_hypercall_on_cpu().

Introduce a new arch_hypercall_tasklet_result() hook which is implemented by
both architectures, and drop the return_reg() macros.  This logic will be
extended in a later change to make continuations out of the tasklet work.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Julien Grall <julien@xen.org>