Jeff pointed out that I missed these in r351742. Also sprinkle
__read_mostly, since these variables are accessed by PHYS_TO_VM_PAGE.
We could go further and make vm_page_array a const pointer on amd64,
i.e.,
```
#ifdef __amd64__
vm_page_t const vm_page_array = (vm_page_t)VM_MIN_KERNEL_ADDRESS;
#else
vm_page_t vm_page_array;
#endif
```
But I don't really like having the inconsistent definition. I will
commit the changes separately.