xen/arm: Allow to discover and use SMCCC_ARCH_WORKAROUND_3

xen/arm: Allow to discover and use SMCCC_ARCH_WORKAROUND_3

Allow guest to discover whether or not SMCCC_ARCH_WORKAROUND_3 is
supported and create a fastpath in the code to handle guests request to
do the workaround.

The function SMCCC_ARCH_WORKAROUND_3 will be called by the guest for
flushing the branch history. So we want the handling to be as fast as
possible.

As the mitigation is applied on every guest exit, we can check for the
call before saving all context and return very early.

This is part of XSA-398 / CVE-2022-23960.

Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Julien Grall <julien@xen.org>