Cleanup `ARCH` handling, reorganize `AArch64` and `Make.inc`.
* Previously, behavior differed if the same value of `ARCH` was defined within `Make.inc` or defined on the command line. Don't do that. * Provide saner defaults for `ARCH` and `MARCH`, and more importantly, allow for the proper overriding of both. * Split `AArch64` code further away from the other `arm` code.
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