mm: make HPAGE_PXD_* macros even if !THP

These macros can be helpful when we plan to merge hugetlb code into
generic code.  Move them out and define them as long as
PGTABLE_HAS_HUGE_LEAVES is selected, because there are systems that only
define HUGETLB_PAGE not THP.

One note here is HPAGE_PMD_SHIFT must be defined even if PMD_SHIFT is not
defined (e.g.  !CONFIG_MMU case); it (or in other forms, like
HPAGE_PMD_NR) is already used in lots of common codes without ifdef
guards.  Use the old trick to let complations work.

Here we only need to differenciate HPAGE_PXD_SHIFT definitions.  All the
rest macros will be defined based on it.  When at it, move HPAGE_PMD_NR /
HPAGE_PMD_ORDER over together.

Link: https://lkml.kernel.org/r/20240327152332.950956-4-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Tested-by: Ryan Roberts <ryan.roberts@arm.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andrew Jones <andrew.jones@linux.dev>
Cc: Aneesh Kumar K.V (IBM) <aneesh.kumar@kernel.org>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: James Houghton <jthoughton@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: "Mike Rapoport (IBM)" <rppt@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Rik van Riel <riel@surriel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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