Unify lazy types (#804)

Replace ad-hoc atomic lazy caches with shared lazy helpers in the
Linux/Android fallback, NetBSD, RDRAND, and RNDR backends. Adds
`LazyPtr` and `LazyBool` so pointer and boolean caches use
the same initialization contract.

This reduces duplicated cache logic and keeps backend probing/fallback
semantics aligned while preserving the existing retry-until-cached
behavior.