[PATCH][ARM64] Add big endian target, port over subtargetfeatures from AArch64

[Tim Northover]

Hi James,

> These two patches add a big endian target to LLVM for ARM64, and port across
> the missing subtarget features for neon, crypto and FP. While we’re there,
> also port over the CPU support for A53 and A57.

Both of them look fine to me.

Cheers.

Tim.