amd64: include opt_kstack_pages.h

This fixes an early KASAN initialization panic in
pmap_san_enter_early_alloc_4k, when a non-default value is specified for
KSTACK_PAGES in the build config file.

Also, rearrange amd64/locore.S's #include order to match the
counterparts of other architectures. And amd64/locore.S now also
explicitly include opt_kstack_pages.h as well.

Sponsored by: Juniper Networks, Inc.
MFC after: 7 days
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D51676