xen/arm: Unmask the Abort/SError bit in the exception entries

xen/arm: Unmask the Abort/SError bit in the exception entries

Currently, we masked the Abort/SError bit in Xen exception entries.
So Xen could not capture any Abort/SError while it's running.
Now, Xen has the ability to handle the Abort/SError, we should unmask
the Abort/SError bit by default to let Xen capture Abort/SError while
it's running.

But in order to avoid receiving nested asynchronous abort, we don't
unmask Abort/SError bit in hyp_error and trap_data_abort.

Signed-off-by: Wei Chen <Wei.Chen@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>