xen/keyhandler: CFI hardening

xen/keyhandler: CFI hardening

Control Flow Integrity schemes use toolchain and optionally hardware support
to help protect against call/jump/return oriented programming attacks.

Use cf_check to annotate function pointer targets for the toolchain.

Tweak {IRQ_,}KEYHANDLER() to use a named initialiser instead of requiring a
pointer cast to compile in the IRQ case.

Reposition iommu_dump_page_tables() to avoid a forward declaration.

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