Make it an error to specify the wrong ARCH_VARIANT

It was previously a warning to set an incorrect ARCH_VARIANT
when CPU_VARIANT was set to a cpu for which we knew the arch.

Make it an error, as there are no offenders in the tree.

Re-structure the code to make it a little clearer when you get
a default and when you get an error.

Test: build_test
Change-Id: I5821a8162a59e8c2aed2370068f9eb656ced9d99