ghc-cabal: Use fromFlagOrDefault instead of fromFlag
ghc-cabal: Use fromFlagOrDefault instead of fromFlag

As fromFlag is partial. The only case where we used fromFlag is when
determining whether to strip libraries; we now assume that we shouldn't.

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