Fix bugs with unsigned rsAtomicCas/Max/Min.

BUG=5888007

rsAtomicMax/Min did not have proper semantics for unsigned integers. They were
always using signed comparisons. rsAtomicCas had the wrong function signature
in our math library, leading to no way to properly link/execute it.

Change-Id: I336cdd8cd9f2d8093f12e101b55b2797515f039b