tools/mm: add thpmaps script to dump THP usage info

With the proliferation of large folios for file-backed memory, and more
recently the introduction of multi-size THP for anonymous memory, it is
becoming useful to be able to see exactly how large folios are mapped into
processes.  For some architectures (e.g.  arm64), if most memory is mapped
using contpte-sized and -aligned blocks, TLB usage can be optimized so
it's useful to see where these requirements are and are not being met.

thpmaps is a Python utility that reads /proc/<pid>/smaps,
/proc/<pid>/pagemap and /proc/kpageflags to print information about how
transparent huge pages (both file and anon) are mapped to a specified
process or cgroup.  It aims to help users debug and optimize their
workloads.  In future we may wish to introduce stats directly into the
kernel (e.g.  smaps or similar), but for now this provides a short term
solution without the need to introduce any new ABI.

Run with help option for a full listing of the arguments:

    # ./t...