Fix strictness signatures of `prefetchValue*#` primops

Their strictness signatures said the primops are strict in their first
argument, which is wrong: Handing it a thunk will prefetch the pointer
to the thunk, but not evaluate it. Hence not strict.

The regression test `T8256` actually tests for laziness in the first
argument, so GHC apparently never exploited the strictness signature.

See also ghc/ghc#8256 (comment 310867),
where this came up.