Make cpu-specific optimizations simpler, smarter, and more compatible with CM

infer what we use TARGET_ARCH_VARIANT_CPU for form TARGET_CPU_VARIANT
if TARGET_ARCH_VARIANT_CPU is undefined (as is the case with a virgin CM
device tree.
(based on 720710b - build: Cleanup CPU-specific optimizations (4 weeks ago) <Steve Kondik>)

assume some things are always true, and remove vestigial variables

Change-Id: I7e5c5809b1dc3f19e99cb153461876077b237345