Use MACHINE_CPUARCH when checking for arm64

[andrew]

Summary

Use MACHINE_CPUARCH with arm64 (aarch64) when we build code that could run
on any 64-bit Arm instruction set. This will simplify checks in downstream
consumers targeting prototype instruction sets.

The only place we check for MACHINE_ARCH == aarch64 is when building the
device tree blobs. As these are targeting current generation ISAs.

[brooks]

Seems fine to me, but tagging @imp for further review.