The segmentation control feature allows a legacy memory segment to
become unmapped uncached at error level (according to CP0_Status.ERL),
and in fact the user segment is already treated in this way by QEMU.
Add a new MMU mode for this state so that QEMU's mappings don't persist
between ERL=0 and ERL=1.
Backports commit 42c86612d507c2a8789f2b8d920a244693c4ef7b from qemu