This adds tests which will validate architecture page table helpers and
other accessors in their compliance with expected generic MM semantics.
This will help various architectures in validating changes to existing
page table helpers or addition of new ones.

Test page table and memory pages creating it's entries at various level
are all allocated from system memory with required size and alignments.
But if memory pages with required size and alignment could not be
allocated, then all depending individual tests are just skipped
afterwards.  This test gets called right after init_mm_internals()
required for alloc_contig_range() to work correctly.

This gets built and run when CONFIG_DEBUG_VM_PGTABLE is selected along
with CONFIG_VM_DEBUG.  Architectures willing to subscribe this test also
need to select CONFIG_ARCH_HAS_DEBUG_VM_PGTABLE which for now is limited
to x86 and arm64.  Going forward, other architectures too can enable this
after fixin...