xen/arm32: Invalidate BTB on guest exit for Cortex A17 and 12

xen/arm32: Invalidate BTB on guest exit for Cortex A17 and 12

In order to avoid aliasing attackes agains the branch predictor, let's
invalidate the BTB on guest exist. This is made complicated by the fact
that we cannot take a branch invalidating the BTB.

This is based on the fourth version posted by Marc Zyngier on Linux-arm
mailing list (see [1]).

This is part of XSA-254.

[1] https://www.spinics.net/lists/arm-kernel/msg632062.html

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Julien Grall <julien.grall@linaro.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>