rts: Explicit state that CONSTR tag field is zero-based
rts: Explicit state that CONSTR tag field is zero-based

This was a bit unclear as we use both one-based and zero-based
tags in GHC.

[skip ci]
sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc