xen/init: Move initcall infrastructure into .init.data

xen/init: Move initcall infrastructure into .init.data

Its contents is constant.

The ALIGN(32) is also dropped.  On x86, there is nothing between it and a
larger alignment.  On ARM, __init_end_efi is between the two, but its sole use
is to fill SizeOfRawData in the PE Section Table, and doesn't require any
specific alignment.

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