hadrian: enable building stage1 haddock for cross ghc

This commit enables building stage1 haddock for cross ghc. Without
this change, hadrian would panic with "Unknown program" error when
building the _build/stage1/bin/cross-prefix-haddock program needed by
the docs-haddock target, even if it only needs to copy from
_build/stage0/bin/cross-prefix-haddock.