s390x: Make z16 machine model known to Valgrind

s390x: Make z16 machine model known to Valgrind

The z16 machine model hasn't been fully introduced to Valgrind yet.  Add
the missing support:

* Add z16 to the list of machine models in each of `libvex.h',
  `tests/s390x_features.c', and `s390-check-opcodes.pl'.

* Starting with z16, the "store facility list extended" (STFLE)
  instruction can write four instead of three words.  Reflect this in the
  STFLE-helper and in the `stfle' test case.

* Pass the new STFLE-bits unchanged, except for the vector-packed-decimal
  facility, which is unsupported by Valgrind.