xen: arm: clear the exclusive monitor on exception return

xen: arm: clear the exclusive monitor on exception return

Otherwise context switching between two vcpus which are contending the same
lock can result in a spurious success.

Our spinlock and atomics code (which we get from Linux) rely on this behaviour
because they use non-exclusive stores for single instruction operations (e.g.
spin_unlock or atomic_set).

This is not required on ARMv8 since eret implicitly clears the monitor.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Tim Deegan <tim@xen.org>