[$] 128-Bit page tables for Arm

The size of a processor's page-table entries directly limits how much
physical memory that processor is able to access. Back in the 32-bit days,
that limit was 4GB, an amount of memory that once seemed nearly infinite,
but which would now struggle to hold a basic AI-enabled "hello world" app.
The expansion to 64 bits on most popular architectures would seem to
have removed those limits now; some Arm systems, for example, can use
56 of those bits to access up to 72PB of memory. So it might be
surprising that the Arm architecture is evolving to support even larger
page-table entries (PTEs). This
patch set from Anshuman Khandual adds support for 128-bit PTEs, but
who will benefit from this capability is not entirely clear.