MFC r307334, r318252: Support write-through caches on arm64

r307334:
Create macros for the MAIR memory attributes. While here add an uncached
memory type, however the VM code still needs to be taught about this.

r318252:
Add the VM_MEMATTR_WRITE_THROUGH memory type to arm64 and use it to support
VM_MEMATTR_WRITE_COMBINING in the kernel. This fixes a bug where Xorg would
use write back cached memory for its graphics buffers. This would produce
artifacts on the screen as cachelines were written to memory.