xen: arm: Factor out psr_mode_is_user

xen: arm: Factor out psr_mode_is_user

This embodies the logic on arm64 that userspace can be either 32-bit
or 64-bit. It will be used in other places shortly.

Note that the logic differs slightly because the original (in
inject_abt64_exception) knew that the kernel was 64-bit and could
therefore assume that any 32-bit mode was userspace. Instead the
refactored code explicitly checks for usr mode.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Julien Grall <julien.grall@linaro.org>