ARM: Implement vcpu_kick()

ARM: Implement vcpu_kick()

If we change something in a vCPU that affects its runnability or
otherwise needs the vCPU's attention, we might need to tell the scheduler
about it.
We are using this in one place (vIRQ injection) at the moment, but will
need this at more places soon.
So let's factor out this functionality, using the already existing
vcpu_kick() prototype (used in x86 only so far), to make this available
to the rest of the Xen code.
Also adjust the perfcounter name to reflect the new usage.

Signed-off-by: Andre Przywara <andre.przywara@linaro.org>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>