[PATCH][ARM] Add support for ARM virtualization instructions.

[Tim Northover]

Hi Matthew,

I think this still needs tidying up (optimised for gatekeeper process
rather than LLVM's commit log again?). It looks like there *are* a
bunch of separate commits here (adding new instructions and fixing
other ones at the same time isn't usually ideal, though if there was a
conflict before could make sense), but not split up like that at all.

The four attached files in particular don't seem independent of each
other in any meaningful way.

Cheers.

Tim.