xen: Implement xen/alternative-call.h for use in common code

xen: Implement xen/alternative-call.h for use in common code

The alternative call infrastructure is x86-only for now, but the common iommu
code has a variant and more common code wants to use the infrastructure.

Introduce CONFIG_ALTERNATIVE_CALL and a conditional implementation so common
code can use the optimisation when available, without requiring all
architectures to implement no-op stubs.

Write some documentation, which was thus far entirely absent, covering the
requirements for an architecture to implement this optimisation, and how to
use the infrastructure in general code.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Daniel P. Smith <dpsmith@apertussolutions.com>
Acked-by: Jan Beulich <jbeulich@suse.com>