powerpc: Add Hypervisor Facility Unavailable handling

Treat Hypervisor Facility Unavailable as an illegal instruction in user
space. This way programs that use facilities like transactional memory,
which will never be supported as it has been removed from the ISA, will
not crash the kernel.

Tested by: pkubaj
MFC after: 1 week

(cherry picked from commit 0b663e0cee89dedf5b070ff4e839d5ad95061535)