Hadrian: fix binary-dir with --docs=none
Hadrian: fix binary-dir with --docs=none

Hadrian's "binary-dist" target must check that the "docs" directory
exists (it may not since we can disable docs generation).

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc