xen/arm: Introduce a macro to synchronize SError

xen/arm: Introduce a macro to synchronize SError

In previous patches, we have provided the ability to synchronize
SErrors in exception entries. But we haven't synchronized SErrors
while returning to guest and doing context switch.

So we still have two risks:
1. Slipping hypervisor SErrors to guest. For example, hypervisor
   triggers a SError while returning to guest, but this SError may be
   delivered after entering guest. In "DIVERSE" option, this SError
   would be routed back to guest and panic the guest. But actually,
   we should crash the whole system due to this hypervisor SError.
2. Slipping previous guest SErrors to the next guest. In "FORWARD"
   option, if hypervisor triggers a SError while context switching.
   This SError may be delivered after switching to next vCPU. In this
   case, this SError will be forwarded to next vCPU and may panic
   an incorrect guest.

So we have have to introduce this macro to synchronize SErrors while
returning to guest and doing context switch. In this macro, we use
ASSERT to make sure the abort is ummasked. Because we unmasked abort
in the entries, but we don't know whether someone will mask it in the
future.

We also added a barrier to this macro to prevent compiler reorder our
asm volatile code.

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