xen/arm: Introduce a helper to check local abort is enabled

xen/arm: Introduce a helper to check local abort is enabled

In previous patch, we have umasked the Abort/SError bit for Xen
in most of its running time. So in some use-cases, we have to
check whether the abort is enabled in current context. For example,
while we want to synchronize SErrors, we have to confirm the abort
is enabled. Otherwise synchronize SErrors is pointless.

Signed-off-by: Wei Chen <Wei.Chen@arm.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>