s390x: Support the PPA instruction

s390x: Support the PPA instruction

The perform processor assist (PPA) instruction provides the CPU with
special execution hints.  It belongs to the processor-assist facility,
which shares facility bit 49 with the execution-hint and load-and-trap
facilities and with the miscellaneous-instruction-extensions facility 1.

Implementing PPA enables setting facility bit 49 to one.  Similar to other
execution hint instructions, implement PPA as a no-op for now.