mm: introduce arch_free_heap_page()

mm: introduce arch_free_heap_page()

common/page_alloc.c references d->arch.relmem_list, which only exists on x86.
This only compiles on ARM because page_list_del2() discards its second
argument.

Introduce a new common arch_free_heap_page() which only uses common lists in
struct domain, and allow an architecture to override this with a custom
alternative.  x86 then provides a custom arch_free_heap_page() which takes
care of managing d->arch.relmem_list.

No functional change.

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