xen/arm: Add ARCH_WORKAROUND_2 support for guests

xen/arm: Add ARCH_WORKAROUND_2 support for guests

In order to offer ARCH_WORKAROUND_2 support to guests, we need to track the
state of the workaround per-vCPU. The field 'pad' in cpu_info is now
repurposed to store flags easily accessible in assembly.

As the hypervisor will always run with the workaround enabled, we may
need to enable (on guest exit) or disable (on guest entry) the
workaround.

A follow-up patch will add fastpath for the workaround for arm64 guests.

Note that check_workaround_ssbd() is used instead of ssbd_get_state()
because the former is implemented using an alternative. Thefore the code
will be shortcut on affected platform.

This is part of XSA-263.

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