build: no more generic kernel toolchains

PS2: fck mac users and fix derp
PS3: add back darwin and clean up
PS4: fix derp
PS5: simplify and fix cpu variant from not sticking
Change-Id: I4a562cde12d75b0c2d7a0ba3f07e300bcff2b136
Store our best guess at TARGET_ARCH_VARIANT_CPU if it is not defined

Change-Id: I850ea59be32cf725b9ca8adb3693f30a42e31d54