rts: Print newline after "Stack trace:" on barf
rts: Print newline after "Stack trace:" on barf

[skip ci]

sdiehl/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc