xen/arm64: Implement a fast path for handling SMCCC_ARCH_WORKAROUND_2

xen/arm64: Implement a fast path for handling SMCCC_ARCH_WORKAROUND_2

The function ARM_SMCCC_ARCH_WORKAROUND_2 will be called by the guest for
enabling/disabling the ssbd mitigation. So we want the handling to
be as fast as possible.

The new sequence will forward guest's ARCH_WORKAROUND_2 call to EL3 and
also track the state of the workaround per-vCPU.

Note that since we need to execute branches, this always executes after
the spectre-v2 mitigation.

This code is based on KVM counterpart "arm64: KVM: Handle guest's
ARCH_WORKAROUND_2 requests" written by Marc Zyngier.

This is part of XSA-263.

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