[$] 128-Bit page tables for Arm

The size of a processor's page-table entries directly limits how muchphysical 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 64bits on most popular architectures would seem tohave removed those limits now; some Arm systems, for example, can use56of those bits to access up to 72PB of memory. So it might besurprising that the Arm architecture is evolving to support even largerpage-table entries (PTEs). Thispatch set from Anshuman Khandual adds support for 128-bit PTEs, butwho will benefit from this capability is not entirely clear.