xen/init: Annotate all command line parameter infrastructure as const

xen/init: Annotate all command line parameter infrastructure as const

There is no reason for any of it to be modified.  Additionally, link
.init.setup beside the other constant .init data.

While editing this area, correct the types used in the extern
declarations for __setup_start and __setup_end to match the types the
linker actually produces.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien.grall@arm.com>