arch-arm: Signal an event when executing store exclusives

arch-arm: Signal an event when executing store exclusives

When a store exclusive is executed, whether it is successful or not,
the exclusives monitor is cleared and therefore we need to signal an
event for the PE.

Change-Id: I383c88c769c0ac5f5d36c4b5d39c9681134d3a20
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/4480
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>