Hadrian: correct deps for ghc builder.
Hadrian: correct deps for ghc builder.

Previously, when needing ghc as a builder, the ghcDeps (Files
the GHC binary depends on) for the current stage were needed.
This is incorrect as the previous stage's ghc is used for building.
This commit fixes the issue, needing the previous stage's ghcDeps.

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