[PATCHTRAIN] ARM64 MC layer fixes

[Tim Northover]

Hi James,

> I apologise that there are so many patches attached, but most are very
> small. I've also attached a monolithic patch for your convenience. There are
> a ton of small fixes and one fairly large refactor which brings over the
> system register implementation from AArch64 to ARM64.

I think these are basically OK. There are a couple of missing
regression tests, but those will be covered by the merge effort and
you're running an even better set anyway.

I think you should go ahead and commit.

Cheers.

Tim.